For a first time that was pretty good.
Here are some things, though:
*In the sequel, you should add an option to customize colors for characters.
*You should add a cheat menu. For the cheaters, not me.
*There should be more enemies, and more weapons.
*Getting killed by a 6 foot drop.
*Jumping should be higher.
*Work on the physics.
*More weapons.
*Too short.
Other than that, it was very well written. I enjoyed it very much. Please make a sequel!