You have to first get all other aeons in the game. Then, you go back to the place where Tidus got attacked by that giant fish thing (I forget it's name) and defeat it again. Though this time it's a lot stronger and has stronger attacks.
After you defeat it, then you'll gain access to a new area. There, you just activate the things and you'll gain access to the fayth's chamber. Anima is yours.
You can also look this up on walkthroughs. They're probably be a lot more detailed.