Use arrow keys to move the spaceship, and press space to shoot the bullet. The buttons are to upgrade your spaceship temporarily, and the green squares show that you can buy the upgrade that it is next to. All you need to do to beat the game is to destroy the enemy ship with the green turret. The clones of it don't matter as much. You start with ten lives, the enemy has 200 lives, and the clones each have 100. If you destroy one clone, all other clones will be destroyed, but not the original.
Decided to take another break from my pen projects.