Ask your ballet teacher for recommendations or any requirements she may have. Some teachers require that their students use a particular kind of shoe for early pointe work. You may also be asked to purchase your shoes in a certain color (although pink is most common) or from a particular store that offers discounts to students from your dance studio. Your teacher may also recommend a specific kind of padding for the toe box of the shoes.
Make an appointment with the fitter in advance; a first fitting for pointe shoes can take up to an hour and a half. Wear your tights when you go for your fitting, so you'll know how well each shoe fits with the tights you normally wear in dance class. Cut your toenails a few days before the fitting.
Try on several kinds of pointe shoes, using the box padding recommended by your teacher or the shop fitter. The shoe should fit closely against the sides of your foot and not gape open on the sides when you flex your foot. If your foot slides inside of the shoe, it is too big. If your foot rises away from the sole of the shoe when you arch it, you need a shoe with a higher arch. In the toe box, your toes should not overlap at all, and the tips of your toes should barely touch the end of the box.
Test each pair of shoes by going to the barre (most dance stores will have one, along with a mirror) and, with one foot flat, putting the other foot up as if you were on pointe. Look for any gaps in the edges or wrinkling around the toe box; both of these indicate a poor fit.
Never buy pointe shoes with "growing room." The pointe shoe must fit the foot as it is at the time of purchase; shoes that are too large can cause technical problems and injuries to develop. Dance students can expect to replace their pointe shoes every one to three months. Professionals often replace their shoes with every performance or sometimes use even two pairs for a single performance.