After you defeat team magma with steven, you must go back to steven's house and he will give you the HM Dive. Then you have to surf south until you reach a certain point(I'm not really sure). Then you will have to use dive and go into a cave and find a submarine which was stolen by team aqua. Dive up and you will be in the team aqua hideout. Hope this helps :)
get a paras. if it's too late, then you train, train, train!
route 25
Some options that could beat Misty's last Pokemon would be either the Razor Leaf, Vine Whip or Solar Beam attacks.
you go to cerulean city and beat misty.
After you defeat Misty, You can go catch suicune by Bill`s house.
Once you beat the Pokemon League at the Indego Platue, you will be able to go to Kanto and see Misty.
beat misty in second gym
Use a electric type or grass type Pokemon to beat her.
This can actually happen. For example your playing Pokemon yellow and you only have pikachu as a Pokemon, you will not be able to beat brock not matter what level pikachu is, but misty can be defeated because pikachu's electric type moves are strong against water type Pokemon.
cerulean city.good day sir
mewtwo in crulean cave after you beat the elite 4. p.s. i don't know how to spell crulean so its the town misty is the gym leader
Once you fixed the Power Plant, go to the end of Route 25. Talk to Misty and she'll return to the gym. Then you can beat her. (She uses water Pokemon so I suggest you use grass or electric Pokemon.)