Ok, first off. M to mute, N to unmute. P to pause, U to unpause. Its written all over the game. NO COMPLAINING ABOUT THAT.
Holding the mouse down is auto-fore. F toggles on and off. the freeze shot(if you have the powerup).
Don't forget to buy stuff at the skills shop after every level. You'll die without the skills.
Fight through 10 levels of alien invasion while upgrading your cannon after each level. Enemies start gaining abilities at Level 3.
Use the mouse to aim and fire and buy skills at the skill shop after each level.
HINT: If you think level 2 and up is too hard, play with the powerups, the multi-fire powerup is crucial.