Here are some cheats for pokemon.
MISSINGNO- Must have fly and surf. Go to Viridian City and talk to the man in the North of town. Answer his question no and then fly to Cinnabar Island and surf up and down the right coast.You should meet MISSINGNO after awhile.
OVER LEVEL 100 CODE- Same code as above.Sometimes you can encounter pokemon over level 100 by doing this. If you have gameshark you can specify the pokemon you get by putting in its code.
LOTS OF ITEMS- Do the same trick as above but put the item you want to duplicate in the 6th item slot before you do that. Fight and run away from a MISSINGNO and you will have over 100 of the item in your 6th item slot.