First go to the options menu, and choose game settings en than select ' cheats ' there you can enter the following cheats:
Unlimited ammo:
L1, L2, X, R2, R1
Invincibility:
up, square, circle, X, down
grenades as bullets:
X, circle, square, L2, R2
Kill enemy's whit one shot:
circle, L2, down, R1, X
Unlock al bonus:
right, R1, L1, down, circle