How to get fly attack in Pokemon ruby version?

You can't get Fly in Pokémon Ruby. It is only available in the following games:

* Pokémon Emerald: You get Fly from Steven Stone after defeating him in the Slateport Gym.

* Pokémon FireRed and LeafGreen: You get Fly from Professor Oak after beating the Elite Four.

* Pokémon Diamond, Pearl, and Platinum: You get Fly from a Pokémon Ranger after beating the first Gym Leader.

There are no other versions where Fly is available.

It's possible you're thinking of a different Pokémon move, or maybe you're playing a different version of the game. If you're sure you're playing Ruby, you can't get Fly.