Define your character, select the game mode and decide if you play with the humans or the aliens. There's 15 campaigns to complete, but only the first one is available when you start.
Arrow keys or W-A-S-D keys to move and jump; space bar to jump. If you jump when you are in the air you'll obtain a double jump. Use the 0-9 keys to select a weapon; or use the Q/E or the ENTER/SHIFT keys to switch weapons back/forward. With the P or CTRL keys you can pause the game. And TAB key to see the scores.