How to drive/pilot/etc.:

Basic controls:
All vehicles are steered by looking around.
You can use the forward button(same as you would use for walking), to move the vehicles, but only some will be able to reverse. For example, cars can reverse but planes cannot.

Boosts:
Some vehicles can have a small boost when the 'use/aux1' key is held. It will only last for a limited time and it will not recharge whilst the key is still held down. 

Weapons:
Vehicles can also use weapons, for example the jet and tank, which will fire a missile when 'sneak' is pressed. They require a missile to be in the drivers inventory to do this. It is possible to have a second weapon, fired with 'use/aux1', but currently only the assault suit does this. It can use both bullets and missiles. The gun turret uses bullets.

Flight/Jumping/Hovering:
Some vehicles can fly, for example the jet. The jet will move upward when the driver looks up, or when the driver presses 'jump'. Using the jump key does not work very well at the moment. The plane is a bit differrent; It will hold it's height when 'jump' is pressed.
It is also possible for vehicles to jump or hover for a small amount of time. Currently only the Assault suit does this.

Boats and watercraft:
The speed boat can be used on water, but if it is driven onto land it will stop completely. If you are lucky you can move back into water, but be careful because this does not always work.

Other things: