If you need help in the game then try this bunch of cheats at the 'Press Start' screen.

Unlock ALL Versus Characters:
Press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left then press circle and start at the same time.

Unlock Level Select:
Press Up, Down, Down, Right, Right, Right, Left, Left, Left then press Triangle and Start at the same time.

Unlock Max Power Mode:
Press L1, R2, L2, R2, L2, Triangle, Circle, Triangle, Circle, hold L1 and press Start.

Unlock Trial Mode:
Press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left then press Square and Start at the same time.

Unlock Sudden Death Mode:
L2, R2, L2, R2, Circle, Triangle, Circle, Triangle, hold L1 and press Start.