These are the controls in a nutshell:

Your clonk has three inventory slots on which two can be selected at the same time. What you have and what you have selected is shown at the bottom. The top shows your clonks. You can click on the stuff in the HUD, just try it out - you will find out what it does. The key controls:

WASD for movement
QE for previous/next clonk
SPACE for grabbing, ungrabbing a vehicle, exiting, entering a building
123 for inventory selection as shown in the HUD
4567890 for selection of vehicles and buildings you are standing in front of

Mouse for aiming and using:
left mouse button for using/throwing the primary selected item
right mouse button for using/throwing the secondary selected item

Drop by holding down (S) while pressing the mouse button. Force throwing by holding shift while pressing the mouse button.