Not bad for first flash game
You've covered the basics for the generic shooter, but it could use some stylistic points, such as music and sound effects. I may also suggest, next time you tweak with something like this, go via a level system and maybe different power-ups besides increased ammo, such as side lasers or shields. Spamming the enemies with seven at a time shots became boring after a while and I quit the game.
Overall, good job on a first game, but needs tweaking. Keep it up.