These are the cheats for that game you play!
Safari trick: First go to the Safari zone and go to the area with the pokemon you are trying to capture in it. Walk around in that zone until your time runs out. When back out side fly to Cinnabar Island and swim on the side that looks like land and water. if you swim up and down you start to run into the pokemon at the Safari Zone and battle them!!!!
Cinnabar trick: Go to Viridian city and talk to the old man with coffie. Say no your not in a hurry and he'll show you how to capture a weedle. Then fly to Cinnabar and swim on the side with land and water. Soon you start to run into pokemon OVER 100!
Missingno.: Do the same exact thing with the Cinnabar Trick and you should run into Missingno. while swiming!
99 items: In order to get 99 items just make sure the item you want to duplicate is the sixth Item down and run into a missingno. (if you do this trick your hall of fame will mess up and may cause you to see Mew!)
Stadium tricks!
Surfing Pikachu:
In stadium beat the master ball prime cup with pikcahu as your 3 through the whole thing and it will replace an attack with surf. Now pikachu will surf on a surf board on yellow and stadium!
Amnesia psyduck:
Get all trophies in the hall of fame.
Battle Mewtwo and round 2
Beat round one, gym leader castle and all cups
Hyper Mode
Beat the game corner, on hard, on who's the best with 5 wins. Beat it straight 5 times and it will give you hyper mode.

The False Tricks! Belive me, there is NO way to get a Mew without gameshark! I have tried all the tricks and they failed.