Save and reset, then chose another player. Switch back and you should have full health.

Before you get Goku, go to the northern mountains. Find the barrier that Goku is supposed to break. There should be a dinosaur behind it. Hit it through the fence. You will gain a lot more if you are under level 30.

When you get the ability to become a super sayian,go somewhere. Turn super sayian and go to the map. And there you go, you're flying around the world as a super sayian.

Get to level 50 with Gohan, Piccolo, Vegeta, and Trunks. Go to the character specific gates marked with 50 to collect trophies. The locations are as follows:

Gohan: Outside Gingertown
Piccolo: New Namek (you must find all seven Nameks to visit)
Trunks: West City
Vegeta: Northern Mountains

After you complete the Cell Games, Hercule will become available.