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She appears as a random encounter in any forest on the World Map. The earliest place you can find her is in the small forest between Fort Condor and Junon. As she is a random encounter, you have to defeat her first (she isn't very difficult and her level will be based on your characters' levels). You may have to fight quite a few battles in the forest before she appears. After you beat her you have to answer some questions she asks correctly, if you don't answer them correctly she will flee and take some of your money. If that happens you will have to find her in a forest again. If you wanna know how to answer the questions correctly use an online walkthrough, because I can't remember them off the top of my head, and I'm not going to search for them for you.

(There's another question around here about getting Yuffie to join the party, I answered that and if you can find it, the exact answers are there.)