You have to go to the lost woods make a left and give it to the guy sitting down, then he gives you the mushroom you have to take it to the old lady behind the potion shop to her potion shop then go to the lost woods again this time instead of that guy there you see a little kokiri girl give her...
Midna is not all bad, I think it is cool all the powers she has I just wished she could have helped me fight some of the harder bosses not just with Ganon which she made that a whole lot easier. I think she is one of nintendo's best work.