What do you do after you give celio the ruby and sapphire on Pokemon Firered for GBA sp?

You will be able to go into Cerulean Cave, for the guy that was blocking the cave will be gone by then. You can find Mewtwo in there. (If you think you can only catch Mewtwo with a Master Ball, here's a tip. Stall and then use Timer Balls. The outdoor shop at Two Island will be selling them. You might need to buy more than one.)