First, you need a disk that can teach Pokémon the move Fly. Second, a Pokémon that can learn the move, usually some sort of bird. Potentially third, you need a badge you can earn by battling the strongest trainer in Violet City, if you happen to be a trainer instead of a breeder. It takes a while, unless you manage to buy an egg that'll know Fly upon hatching.

[Yes, he's quite aware that this'll just lead to more questions. He's ready.]

I had a Houndour. They're little hellhound puppies that can breathe fire, and their fangs would make you glad you're just getting pinched. Very loyal and friendly, though.