try to find yourself inside there,
You need surf and some other pokemon to get to mewtwo
It is level 70 and the highest in the game(if not counting your own pokemon)It is very hard because it uses recover to heal itself and psychic(followed by its very high special)can maul almost any pokemon, if it does not resist it.Also, to get in, you need to beat your rival and claim the title champion.It is left of the bridge north of cerulean city.You need to surf down to find the cave, and if you have not beaten your rival the last time, there will be a guard that will not let you pass
I apologize for my long post.