If you are a new pointe student it is best to visit a store and be professionally fitted for the shoes. Mail order can be useful later on when you settle on a brand, style and size.
Always talk with the teacher about how she wants the shoe to fit and what padding to use. If possible go to the store with your teacher or an older dancer from the studio. Having someone with experience at the first fitting is very important since parents sometimes don't quite understand the way the shoes should fit and the fitter may not understand the way the teacher wants the shoe to fit. If your teacher can't go try to use the fitter she recommends and always tell the fitter what the teacher's name is.
Most pointe shoes are handmade so the sizes can vary a little even between different pairs of the same shoe. Never go to a pointe shoe fitting empty handed. Take your 'teacher approved padding' along to be sure the shoe fits like you'd wear it in class.
Note: Before you head to the store make sure you've trimmed those toenails! Make sure they're not too long or too short. Do not cut them the day of the fitting! Cut them a few days before just in case you cut them too short. Make sure you don't have any sharp edges that might poke into your skin. As a pointe student you must take proper care of your feet and toe nails are the first step.
Be prepared to stay at the store for a while because the fitting can sometimes take hours. You'll probably try on many different pairs before deciding. The fitter will start by asking for street shoe size or ballet shoe size. The fitter should look at the dancer's feet, the length of their toes, the shape of the foot, and take this all into consideration before deciding on which shoes to try.
Each part of the pointe shoe has a name. Try to familiarize yourself with the different parts of the shoe before you head to the store for the first time. That way you'll know what the fitter is talking about if she asks "Is that vamp too high?"

(a) inner sole / sock liner
(b) shank
(c) drawstring / drawstring knot
(d) wings
(e) box
(f) drawstring case
(g) leather outer sole
(h) vamp
(i) platform
Sometimes the feel of the shoes can be a bit overwhelming to the new student. One important point to remember is that the shoe should not allow "growing room." The shoes should fit perfectly. A shoe that is too big could ultimately hurt the dancer.
Next, with the shoes on, go into second position and plie. The tips of the toes should barely touch the inside of the box. Then put one foot up in a pointe position (as if going up on pointe) with the other foot still flat on the floor and no weight on the pointed foot. If the pointed foot slides too much into the shoe when this is done, the shoe is too wide. If the foot seems to be arching out of the shoe a higher vamp may be needed.
Finally, stand on releve in first position. Using a barre or an adult for support allow the fitter to take a good look at the shoes. Some dancers will not be allowed to do this if a strong arch is detected because the shank can be broken in the store.
Once the perfect shoe has been selected pick the right color and width of ribbon. I suggest the wide 7/8" ribbon because it provides more support to the ankles. The 5/8" ribbon does make the dancers feet look smaller and sometimes the thick ribbon looks ridiculous on a dancer with small legs and feet.
If suggested by the teacher also select your elastic. Make sure enough of the elastic is cut. Sometimes, in shoes that do not have a drawstring, it is necessary to cross the elastics, meaning you'll need more. If the elastic is to be placed across the top of the foot I suggest the wide elastic. If the elastic is to be sewn on the heel of the shoe then the thin elastic is needed.