How to walk in Tony Hawk Underground?

This is the 1st one I believe. Or it could be the 2nd because the game store that I got it off can be a bit stupid and put the wrong disk in.... So the cover says Tony Hawk Underground, but I can walk in this game, and I don't know how to. Can someone PLEASE tell me how to walk, it's really bugging me... Please tell me properly... Thanks. It's on PS2