At each level, the gun seller will tell you what mission you need to fulfill; but it's not necessary that you read all the instructions: basically it's about going forward killing all your enemies until you reach the end, using the many weapons and commands provided by the game.
Use the W-A-S-D keys to move. Move the mouse to control your direction of attack: click to attack. Arrow keys 1-2-3-4 or E to switch weapons. R key to restart. G key to release your weapon. Q key to see info about your current situation. Space bar to communicate and drive. A right click during the game allows you to send commands to your teammates or save the game.