It can't get crazier than this!

Follow the in-game instructions.

Use WASD for jump, left, duck, right respectively.
Use the MOUSE to move your direction of fire and MOUSE WHEEL or numbers 1,2,3,4,5 to change weapons. Hold down SHIFT whilst moving to run.
Use SPACEBAR to look around. Double-tap duck to jump down from a platform.
Press P to view the current mission. More controls explained in the game.