Can you trade pokemon from ruby to leaf green?

Is it possible to trade pokemon from Pokemon Ruby to Pokemon Leaf Green and if so how

Go to the pokemon center. Its somewhere up stairs. I don't have the game anymore so I'm not 100% sure.

Yes, but you mut beat both games first.

if you want to trade from leaf green/fire red to ruby/sapphire or what ever you have to get the ruby and sapphire(ingame items) on leaf or fire AND you have to have both games and 2 GBAs. I am not totally sure where they are but they are both found in different caves (you will have to figure that part out yourself hehe)
have fun