How do you pick your character in Marvel Ultimate Alliance 2?

i just started story mode and dont want to be iron man... how do i change it?How do you pick your character in Marvel Ultimate Alliance 2?
ok if ur at the begining u will have to finish the first level once u r done with that go to the blue circle after the cut scene of spider man and wolverine being instructed to do something.at first there are only half the playable characters if u would like to unlock them all enter this code on the character select screen up up down down left left left right.

make sure to type int ill all dark shadowed characters appear then save game to keep characers.