That is a good question. I'll give you the answer.

First you have to go to the Centra Ruins. (Place where you get Odin.) I would suggest doing this after you get Odin. While you are there you have to defeat approximately 20 Tonberrys. If you need help defeating Tonberrys just ask. Then after you defeat the Tonberrys, the Tonberry King will appear. (Immediatly after the battle. You won't exit the battle.) Be prepared to fight. Tonberry King can be tough. If you need help with him just ask again. Once you defeat him he is all yours.