While you are at the Main Menu you can press:

L1, R1, R2, SQUARE, CIRCLE.

When you enter the game you can select your vehicle and then choose any track you would like.

While you are at the Main Menu you can press:

L1, L2, R1, R2, CIRCLE, SQUARE.

When you enter the game you will have every vehicle to choose from.

While at the Main Menu you can press:

L1, R1, R2, L2

This will unlock the next Chapionship. Keep entering the code to unlock more chapionships. A total of 7 more championships can be opened.