This guide only covers some basics about the game. It won't go into great detail and teach you how to be brilliant at every class! I'm only going to be able to pass on some knowledge that I have, and not all of it, as well as some mildly opinionated ideas about parts of the game.

Also, this guide tends to have it's sections taking up a large amount of space each. You have been warned

First of all, you'll need to know the basics of playing the classes. There are 4 which are common to each army (the Republic, the CIS, the Rebellion and the Empire). These fall under the four following categories:

The Commando - a standard trooper perfect for beginners with first-person and third-person shooters. Armed with a high-fire rate weapon, a pistol and grenades (or Thermal Detonators) which stick to vehicles if they land on them. The only exception is the Super Battle Droid on the CIS, who is armed with a shotgun-like wrist blaster in place of the pistol and wrist rockets in place of the grenades. Ideally, you should use this to get used to the controls, and try and take on infantry at about mid-range, switching to the pistol only if you run out of ammo and using your grenades to destroy vehicles or scatter enemy groups.
For each faction, these would be:
Clone Trooper (Republic)
Super Battle Droid (CIS)
Rebel Soldier (Rebellion)
Stormtrooper (Empire).

The Demolitionist - a heavy-duty trooper armed with a pistol and Thermal Detonators, but it's main weapon is a rocket launcher which can lock onto vehicles and home in on them. It's generally quite slow to reload, but can be useful if you keep your distance, and can also be used to take down groups of enemies if played correctly. A Demolitionist would also be armed with Land Mines as a secondary weapon, which can be switched with the Thermal Detonators. Note that they will have less Thermal Detonators, however. The Land Mines which you can place will detonate whenever they detect an enemy player, but Engineers (to be discussed later) can walk over Land Mines without detonating them
For each faction, these would be:
Heavy Trooper (Republic)
Assault Droid (CIS)
Rebel Vanguard (Rebellion)
Shock Trooper (Empire)

The Sniper - a light-armoured class focusing on the classic long-range Sniper Rifle for it's main weapon. Snipers also have a further zoom than their allies, as well as the standard pistol and Thermal Detonators. Snipers are also armed with an Auto-Turret which they can deploy at will as a secondary weapon. Though small and fairly weak, it can be a great way to act as a warning or to protect you from troops who are sneaking up on you.
For each faction, these would be:
Clone Sharpshooter (Republic)
Assassin Droid (CIS)
Rebel Marksman (Rebellion)
Scout Trooper (Empire)

The Engineer - a specialist, short-range support class armed with a unique arsenal of weaponary. A Shotgun which fires bullets in an arc are the main weapons of Engineers. Using these at closer range to opponents deals more damage, but foes who are more distant take less damage, or no damage at all. Engineers are also armed with Fusion Cutters, a high-tech repairing tool for vehicles and turrets which can also "slice" into enemy vehicles, disabling them temporarily and forcing those who were in them out of the vehicle (a great way to steal enemy vehicles). They are also armed with Detonator Packs (or Detpacks), which can be detonated at any time. However, you may only place one Detpack at a time, so you must activate the one you last placed before setting up another. Engineers also have Health and Ammo Dispensing, which allows them to deploy disposable ammunition and a bacta-tank, which heals a fair ammount of health and can restore weapon ammo. It will not, however, restore the Health and Ammo Dispensing Ammunition. You may still use the item on yourself, or on allies to keep them alive or their ammo stocks up. Engineers can also, as previously mentioned, walk past enemy mines without detection.
For each faction, these would be:
Clone Engineer (Republic)
Engineer Droid (CIS)
Rebel Smuggler (Rebellion)
Imperial Engineer (Empire)

8 and 12 point classes - all factions also have 2 unique classes with abilities specific to them. They can vary greatly in what they do. Some have abilities which benefit nearby team-mates, such as Rally, Rage or Regeneration. Some have "Recon Droids", which are small, low-health droids that can scout for info and self-destruct at will. But there are no common traits to these classes apart from the following things:
Each of them require a certain number of points to unlock (8 or 12)
Each of them have a limit to the number you can have on your team (generally 3 or 4 maximum)
Each of them are extremely difficult to beat without the help from other 8/12 point classes or Leaders

Leaders - these are the Jedi, Sith and major characters that appear throughout the Star Wars Universe. They vary from movie icons like Master Yoda and Darth Vader to lesser known characters such as Aayla Secura and Ki-Adi Mundi. Not all Leaders are Jedi, however. The exceptions to the rule of the leader being a Jedi or Sith are as follows:
Jango Fett (CIS)
Boba Fett (Empire)
Princess Leia, Han Solo and Chewbacca (Rebellion)

As for Jedi/Sith Leaders, they are as follows:
Obi-Wan Kenobi and Yoda (available for both Rebellion AND Republic)
Emperor Palpatine (available for both Empire AND CIS)
Mace Windu, Ki-Adi-Mundi and Aayla Secura (Republic only)
Luke Skywalker (Rebellion only)
Darth Maul, Count Dooku and General Grievous (CIS only)
Darth Vader and Anakain Skywalker (Empire only)

Leaders live on a time-limit represented by a light-saber where the health bar would be. They have unique weapons and powers, and in the case of Jedi, Force Powers. Most are capable of jumping multiple times, with the exception of Greivous, Han, Chewbacca and Leia. Jango and Boba may also be considered exceptions, but are instead armed with a Jetpack. The Leader's time limit is affected by a few things:
Number of enemies they kill (increases your time limit)
Number of allies they kill (decreases your time limit)
Number of hits they've recieved (decreases your time limit)

Leaders can be very difficult to take out, especially Jedi/Sith leaders, so my advice is this - if you see them, try and destroy them, but try and stay out of their way! As for playing as a leader, try and get as close to them as possible, since leader weapons are generally short-range or lightsabers.

Space Battle Classes
In space battles, there are two main classes - Pilots and Marines.
Pilots are designed with vehicle usage in mind, focusing less on the weaponary of the infantry and more on their vehicle's power. They're armed with a pistol, time bombs and a Fusion Cutter, and are capable of repairing their ship whilst in flight.
Marines are designed with the idea of combat on foot and internal demolition or defense in mind, thinking of what would happen if a player landed in the enemy lines. They are armed with a rapid-fire weapon, a rocket launcher and thermal detonators, each with finite (limited) ammunition. The idea with this class is to fly into the enemy base, destroy their Engine Cooling Tanks, Auto-Defense Mainframe and Life-Support Systems, and possible Shield Generator, from the inside of the ship. You will, however, encounter some rather heavy resistance when you first land.

There are numerous different gamemodes in Battlefront II, ranging from Conquest to Hunt to Assault. Oh, and there's Capture the Flag, but nobody plays that too often.

First of all, Conquest. It's a fairly simple game - first team to a certain number of kills, wins. Each side will generally start with 150 troops as a limit, and each time a troop dies, this lowers the score. The goal is to lower the opponent's score to 0 before they lower yours to 0. You can also capture command posts to try and limit the ammount of places your opponents can spawn in and earn more for your allies to spawn in. If troops can't spawn in, then you are eventually going to struggle to win. You may also capture command points to take victory, as if you can take all command posts on the map and keep them for 20 seconds after capturing the last one, your team wins by default. So capturing the command points is important. It should also be noted that this is the only gamemode where you are able to capture command points.

Next up, CTF, which, from my experience, is very rarely played. Simply find the enemy flag and take it back to your base. The goal is to capture the flag a certain number of times, usually 3 times. If you have the flag and are playing as a Jedi/Sith leader, or a character/class that has a Jet Pack, you won't be able to use your secondary jumping ability (Jet Pack or Force Jump), although you can still jump. There are 2 types of CTF - 1-Flag and 2-Flag. The difference is simple. In 1-Flag CTF, there is a neutral flag placed on the map, generally near the centre, and the goal is to capture this flag a certain number of times. In 2-Flag CTF, it is similar to Team Fortress, in the sense that there are two flags - one for each team, which need to be captured.

Now for a gamemode which can only be played on specific maps - Hunt. This gamemode features two sides - one alien race, and one faction, with the exception of Mos Eisley, which has two different alien races. The idea is simple - only a certain class or number of classes are put onto the faction's side to account for two things: 1) The alien race will only have a limited number of classes. 2) It's trying to stay realistic to the movie trilogy, in a way. The only exception to this rule is Hoth, in which the Rebels have all classes available to them. Other maps where this gamemode is playable include Mos Eisley, Jabba's Palace, Endor, Naboo and Kashyyyk (there may be more, but I'm not certain). All you have to do on this gamemode - kill each other.

Last, but not least, is the Assault gamemode, which is the space-battle gameplay mode (although CTF is strangely playable here as well). Assault involves the use of points amongst your team to calculate an overall score, and the first team to reach the score limit wins the game. You can score points by killing enemies, destroying enemy vehicles, attacking the enemy frigates and key ship systems, and basically obliterating the opponent. There is also an Assault game-mode for land battles on Mos Eisley, in which you can play as the previously mentioned Leaders with no time limits on them - instead, you have a health bar which is extremely high. You may play as any of the Leaders as well, depending on which side you're on - Heroes (play as Rebellion and Republic leaders) or Villains (CIS and Imperial leaders). Personally, Mos Eisley Assault is my favourite game mode!

In Star Wars Battlefront II, there are a lot of different vehicles, ranged from light-armoured Speederbikes to heavily armoured tanks. Generally, they have the same controls and set-up, often armed with higher powered weaponary and more armour than those of the troops. They can also be faster in moving around the battlefield.

However, in Space-Battles, or Assaults, it is generally important to know about the different vehicle types. You have the choice of 3, or on some maps, 4 vehicles. The types of vehicles are as follows:

Scout class - these are lightly armoured and designed for dogfighting. They can be effective counters to bombers trying to destroy critical systems.
For each faction, they are as follows:
Republic Starfighter (Republic)
Tri-Fighter (CIS)
A-Wing (Rebellion)
TIE Interceptor (Empire)

Multi-purpose class - these have slightly stronger weapons and slower movement speed than Scout class fighters. They're the core fighters of all the different factions generally. They can be used for the same purpose as Scout fighters, but can also easily fulfill Bomber jobs.
For each faction, they are as follows:
ARC-170 (Republic)
Vulture Droid (CIS)
X-Wing (Rebellion)
TIE Fighter (Empire)

Bomber class - these are heavily armoured and slow moving, with bomb-like projectiles which are somehow affected by gravity in space (because screw logic). They are useful for anything that requires some heavy firepower to destroy, like frigates, shields and key systems on the ships.
For each faction, they are as follows:
V-Wing (Republic)
CIS Strike Bomber (CIS)
Y-Wing (Rebellion)
TIE Bomber (Empire)

Gunship class - these rarely show up, but when they do, they can be very useful. They quite simply are the most heavily armoured of all the ships, can hold 4 troops at a time to fight on your behalf, can act as Command-Points for your allies, but are the slowest and can't do any special manouveurs like barrel rolls (insert Starfox reference here). Ideally, you should try and pilot these into the enemy's ship with marines ready to protect it, although you should play as a pilot to actually get anywhere in that ship.
For each faction, they are as follows:
LAAT Gunship (Republic)
Droid Gunship (CIS)
Rebel Gunship (Rebellion)
Imperial Shuttle (Empire)

I might not have covered all the details of the specialist 8/12 point classes or land vehicles, but they aren't my speciality. All you really need to know for basic info about the main classes that has been covered. From here, you must choose your own path - pick out a class which you think is best suited for your style of gameplay, and try jumping onto "Instant Action" in the single-player menu to try out the different classes in practise. Don't worry if you aren't that good with the class you choose to try and play as at first - it's not easy when you first try it.

If you want an honest suggestion, try completing the 501st Campaign that is available. You could also try going back into the main training video to try and get used to the gameplay. Once you've got the basics down, it's important to try and expand on at least one class and your abilities with it. But NOT an 8/12 point class, as this can hinder your team-mates. You need to actually find a way of racking up the points before you can even GET the class you so desperately want, or even the leader you may want to play as.