You will find her at the Cape on Route 25, Which comes after Route 24 the Golden Bridge.
Before You find her, expect seven or eight trainer Battles, including the Last Team Rocket Battle. He will only be a grunt, who does not know that Team Rocket has disbanded again.
On the bride on route 25
hes in a house were you find misty the gym leader for the first time.
Go to the place where you find misty and her boyfriend
You can do the gym leaders in any order you want, although from the Fire Red/Leaf Green point of view the 6th gym leader would be either Misty or Erika.
In her Gym
defeat brock blaine misty ericka poison leader and all the other gym leaders
(Sorry i though for a second you said "Why do you have to find Misty") (This question needs to be improved)
You don't, dummie. Have you even played a Pokemon game before? Misty is only in the Kanto region. She is Cerulean City's Gym Leader, and only travels with you in the anime. You will never find her, period. Go out and learn something.
You need the second badge which is obtained by beating cerulean cities gym leader misty.
She is in Cerealen City.
You can't find her anywhere in the game.
Once you beat the Pokemon League at the Indego Platue, you will be able to go to Kanto and see Misty.