One of the things players should prepare for is the spread of the Hallow and Corruption/Crimson. As soon as Hardmode starts, two diagonal strips of Hallow and Corruption/Crimson will generate in a V-shape, and the two biomes start to slowly consume the world.

The main threat of the infection is its effects on the Jungle. Mud Blocks can be affected by Corruption/Crimson (but not Hallow), and they will be permanently turned into regular Dirt Blocks. Although it takes an extremely long time for it to consume a large part of it, the damage is very hard to repair as it requires the player to manually replace all the Dirt for Mud. A large, 3 block wide tunnel filled with uncorruptible blocks will protect the biome. Note that smashing a Demon Altar has a chance to corrupt or hallow a random block on the map, which could be inside of the Jungle.

The player will need the best gear available in Prehardmode if they want to survive for longer before obtaining better items.

• Melee users should use Molten armor and weapons like Night's Edge, Dark Lance or Cascade.
  
• Ranged users benefit from the Necro armor. The best weapons available for them are the Phoenix Blaster or Molten Fury.
  
• For magic users, Jungle armor or the Wizard set are the best available, with weapons like the Flamelash, Water Bolt or Demon Scythe.
  
• A Summoner's only choice is the Bee armor with the Imp Staff.
  
• All classes should also bring accessories like the Lightning/Frostspark Boots, Shield of Cthulhu, Worm Scarf and one of the Emblems dropped by the Wall of Flesh.
  
• Additionally, a Molten Pickaxe or Reaver Shark is required to mine Cobalt or Palladium ore.
  
• Rare drops like the Black Lens, Nazar or Bezoar are easier to get without the stronger enemies of Hardmode.

A house in the Jungle should be made for the Witch Doctor, as he will sell Leaf Wings when he is in the biome. A King Statue can also be used to simply teleport him to the Jungle.

The player should make a Surface Mushroom biome with a house for Truffle.

A Hellevator should also be built, as it's an effective way to access any depth at any time, which will save a lot of time when mining the new ores and farming for rare drops.

The player can also spend a long time fishing for Crates before fighting the Wall of Flesh, and then open them once it has been defeated to get the new ores. This process is much more time consuming than smashing the Altars with the Pwnhammer and mining, however it is a lot safer since the player will start with the best armors available.

The first thing to do in Hardmode is to go to the natural Corruption/Crimson Biome and start smashing Altars.

• When smashing one altar, the world will be blessed with Cobalt or Palladium Ore;
  
• Smashing another one will make Mythril or Orichalcum to appear;
  
• Smashing a third one will make Adamantite or Titanium to appear.
  
• Smashing every set of three after this point will cyclically generate the same ore, but much lesser than the last, so smashing 9 to 12 will be more than enough.

It's difficult to survive in this new environment, since the new enemies will quickly overwhelm players equipped with even the best Pre-hardmode gear. Because of this, it's better to look for a new weapon before going down into the Caverns to mine the new ores.

There are many different choices for every class:

• Melee users can get an Amarok very early on. It has a low chance to be dropped by any enemy in the Snow Biome, so with a quick farm they can easily obtain it. The Swords or Spears made from ores can be quickly obtained and they will get the job done until better weapons are acquired.
  
• Ranged users can buy a Shotgun from the Arms Dealer as soon as Hardmode starts, which can then be upgraded to the Onyx Blaster. The Clockwork Assault Rifle is also very strong and should definitely be used if the Wall of Flesh dropped it. A Cobalt or Palladium Repeater also gets the job done until the better ones are acquired.
  
• A Magic user can quickly farm for the Sky Fracture, a weapon that is very useful even after Plantera is defeated. They can also fish for a Crystal Serpent, but it can take longer to obtain. The Meteor Staff is even easier to craft, however it is hard to use it underground.
  
• A Summoner will have to get the Spider Staff. This is a problem because the Black Recluses can very easily kill a player in 3 to 4 hits. Since they can't go through blocks, it is recommended to farm the Spider Fangs inside of a "box" while the minions kill the spiders from the outside. Make sure to have your Magic Mirror ready in case a Giant Worm or another enemy spawns.

With these new weapons and a couple Mining and Spelunker Potions, getting the Hardmode ores will be a much easier task.

The Mechanical Bosses can be very hard to kill for a new player, so using the best weapons, armors and accessories is advised.

Palladium and Titanium armors are very good with their defensive set bonuses, while Adamantite and Orichalcum armors are better for those looking for the maximum DPS. It is better to skip Cobalt and Mythril armor, as they don't have very good stats or a special set bonus to make up for it. There is also Frost and Forbidden armor, but they should only be used if the player wishes to mix different damage types, as they aren't strong enough by themselves.

If the player hasn't obtained them already, Wings are essential for the fights, as it's hard to dodge their attacks without them. The best ones available are the Frost Wings, but the Leaf Wings are easier to obtain at the cost of some speed.

An Emblem of the player's class is also important, as well as defensive and mobility accessories like the Cross Necklace, Philosopher's Stone, Ankh or Obsidian Shield, and Frog Leg. Having good modifiers like Lucky, Menacing or Warding is also very important for the battles.

Source[terraria.gamepedia.com]