There's already in-game instruction on how to play, but just as a refresher...
W - Move Up
S - Move Down
A - Move Left
D - Move Right
Aim with the mouse!
LMB to fire, RMB to reload.
Space - after game over, press to take a screenshot of your score
Q - after game over, press to go back to the main menu
E - after game over, press to restart the game
LMB to get out of the extra sections.