Jessica,
Discontinuing medications such as Topamax should be done under a doctor's supervision. They should not be abruptly discontinued since doing so can cause serious problems including seizures. Call or make an appointment with your doctor to discontinue the Topamax. He or she will give you a schedule to taper down, reducing a bit each week. Usually, they have you reduce your dosage by 25 mg per week until you're down to none.
You're wise to be thinking about this before becoming pregnant. If you haven't seen this article already, you should take a look at Migraine Preventive Topamax May Cause Birth Defects.
Good luck!
Teri