there are a few ways to make health bar.. my favorite:

1. draw a long rectangle
2. make it a movie clip
3. tween the inside color to make it look like the health is decreasing..
4. at the end of the movie clip where the health runs out.. tell it to go to "fail"
5. at beginning.. tell it to stop.. or else it will just play and not work.
6. where you code collisions and hits to your character.. tell the health bar movie clip to advance one frame..

check out how it works in this game i made
http://blueco.ws/ShootingGame