First of the maverick can only hold 6 shots. Second, animate your characters. Third, make npcs

This game is on part with an Atari 2600 game, and that's not a bad thing. It's simplicity is it's strong point, in that it has it's own spin on a classic idea.

The two things I'd really work on would be creating multiple enemy types(such as enemies that sway as they walk, move at a different pace, or take more than one hit before they go down) and incorporating the game's lose state a bit better. Since you took the time to add a brief story, you should a game over screen that displays your score and then explains the consequences of the zombie grabbing you or walking off the left side of the screen.

Besides the fact that this is your first Flash game I really enjoyed it, for a little while.
But you need to understand that a simple thing like this alone will not do, you could have many other features like a shop where you could buy new guns, have NPCs to help you, character upgrades, or even modify your guns i.e. you have better rate of fire but decreased damage.
Doing this by yourself will be dificult, you should contact someone to help you, I hope you take some of my ideas, or yours and make another one, i'd love to se a sequel. Keep up the good work! :)

I'll give you 5 stars so this game becomes a little more popular, and maybe you can team up with someone. Someone with experience, to help you practice or give you some helpful tips. This is pretty good for your first flash. Good luck with your games! ^_^

I really loved the game :D
The music was awesome and the story-line was interesting, you should team up with a graphics designer and practice some more difficult stuff so you could make better games in the future. Really nice for a beginner level build :)
The fact that the people slid across the screen was a bit weird but with practice you will master the walk movements also :D