Most faction blueprints and purchasable pack vehicles are garbage.
The random vehicle assembler will most likely only assemble garbage.
Downloading a vehicle from the exhibition is an option, but it pays to know how to make your own vehicle from scratch... that isn't garbage.

When building, use the following guidelines, and you'll be one step closer to being a formidable wasteland survivor :)
(This guide doesn't contain everything, obviously... just some big mistakes that I often see noobs make.)

Fuel rewarded at the end of a match does not stack if you have multiple Fuel Barrels.
The same applies for Fuel Tanks.
You CAN, however, use one of each to get a maximum of 15 fuel per round.

Explosive parts such as fuel barrels/tanks, ammo boxes, and generators need to be protected.
Usually, the BEST place to put explosive parts is underneath your cabin. Just make an open spot in your frame and fit the part in there.
The second-best place to put explosive parts is behind your cabin. If you have to do this, be sure to protect the part with armor.

Mixing weapon types is often a very inefficient use of your energy.
For example, don't use machine guns and a cannon on the same vehicle. Either use all machine guns, or all cannons.

Every cabin provides you with a base amount of energy.
Generators can give you extra energy if needed.
ALWAYS use ALL of your available energy. Any energy not used is a complete waste.

Mount weapons either on your cabin, or on shoot-through parts.
Mounting weapons on the cabin means you have a lower chance of losing that weapon, since it's not mounted on a piece of armor that can be easily destroyed.
Alternatively, you can mount a weapon on "shoot-through" parts, such as frame parts or gun mounts.

This sounds counter-intuitive, because shoot-through parts often have very low durability.
But they also allow 90% of damage to pass through them, meaning that a shoot-through part with 30 durability effectively behaves like it has more like 300 durability.
That being said, mounting weapons to shoot-through parts is a good way to lower the chance of losing that weapon.

Protect weapons by placing armor around them. This further lessens the chance of losing them.
You'll notice that surrounding weapons with armor may limit their firing angle. This is often worth the extra protection.

Never use modules you don't need.
For example, machine guns and shotguns have unlimited ammo, so they don't need ammo boxes, but they do benefit from radiators and coolers.
On the other hand, cannons require ammo boxes, but do not benefit from radiators or coolers.

Tonnage is different from mass-limit.
If you exceed your vehicle's mass-limit, you are prohibited from using it in battle.
However, while exceeding vehicle tonnage doesn't prohibit you from using the vehicle in battle, it has serious performance consequences.

The "Tonnage" stat indicates what the vehicle's tonnage is in relation to its mass limit.
The entire bar is the mass limit, while the little red line is the tonnage.
If your vehicle's mass is greater than its tonnage, the vehicle's mass number will be orange, and your max speed and acceleration will decrease exponentially depending on how much heavier you are than your tonnage allows.

You are given a base amount of tonnage depending on what cabin you are using, but tonnage is further increased by movement parts and some engines.
That being said, if your vehicle's mass is in the orange, you can either reduce weight, or increase tonnage by adding more wheels.