How do you transform into super sonic in sonic heroes

You have to fight Metal Overlord to be Super Sonic. In order to get to him, you have to get all seven Chaos Emeralds and finish all the teams' stories; this will unlock the Metal Overlord, and the story he is in.
only in the final stage u play as super sonic where u fight with metal overlord to get it u must get the 7 chaos emeralds and finish all 4 characters story's

Well, actuatly you can't. you have to play the last story and battle metal sonic, and during the battle you will turn into super sonic.
but if you mean in story mode i think you have to hack your pc/ps2/gcn or you can unlock super hard mode by getting everything in the game (this is hard)