It's not bad for your first flash game, but it is far from finished.
First off, the graphics are very simple. Sure, they're 3D, but there's no details, and the enemies don't look like enemies at all, rather like blocks. Moving blocks.
There is absolutely no sound, and the enemies don't shoot back. the hit detection is good, though. Maybe you could've added a gun, like in most other FPS games? Make the player see the gun you'r eholding and see it firing?
The enemies don't even sh oot back. That's a big issue, and it just loops, it's far too short.