If you play through story mode you will get this along the way.
refer to http://www.gamefaqs.com/console/xbox360/file/928296/39474
if you get stuck along the way.
also if you are still have troubles, use these cheats to make the game a breeze.
Go to the options menu, and then select Cheat Codes. Type in this code.
grindXpert - Helps you keep balance when grinding
h!tchar!de - Hitch a ride on the back of cars
2them00n - Moon Gravity
2wheels! - Perfect manuals