First, make sure you have a heart scale (you can find them underground). Next, fly to Pastoria city. After that, go to the house close to the water, (the one with the guy that says he's exactly in the middle of the city in front.) go inside, talk to the guy inside, if you have a heart scale, he'll teach one of your pokemon a move that it forgot (or never learned)! You have to give the guy a heart scale in return, though.