There's already in-game instruction on how to play, but just as a refresher...

Movement:

W - Move Up

S - Move Down

A - Move Left

D - Move Right

Shooting:

Aim with the mouse!

LMB to fire, RMB to reload.

Other:

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

Menu:

LMB to get out of the extra sections.