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.