At the "Dark Side Of The Moon" menu, press Select. Then, press L, Right, B, L, R, Left to unlock the "Build Secret Base" option. Enter the "Build Secret Base" screen, then press A and enter one of the following passwords:

Invincibility

Enter "GOD" as a password.

All weapons

Enter "WEAP" as a password.

Infinite ammunition

Enter "AMMO" as a password.