Catch Mew
You need a Game Genie (only avalable for NES SNES and Game Boy) for this if you don't have one you can probably only buy one online.
Anyway enter the code 151-91A-7FC then go to the patch of grass west of Cerulean City. There you will find a level 10 mew among common pokemon.
NOTE:Works also for red version
NOTE:Mew will sometimes be a ditto esp. in blue version
Theres loads more info this game, right here, with some cheats and tips, answers and Pokemon Blue walkthrough.
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