Do some research on different brands of shoes -- there are many to select from. Narrow down which brands or styles you would like to try and find a store that carries them. Be sure there is a professional in the store who is adept at sizing ballet shoes. Usually, a dance wear store will carry a full line of brands.
Have the store clerk measure your foot with a measuring tool -- usually a Brannock Device -- to first get an accurate measurement of your street-shoe size. Depending on the brand you select, ballet shoes and pointe shoes can run from one to one-and-a-half sizes smaller than your street shoe.
With each shoe you try on, stand up and walk around. Notice if the shoe pinches your toes. There should be enough space to allow you to comfortably point your toes. As well, notice if your heel is slipping out of the back. In most cases, you will need to sew ribbon or elastic to your shoes, but the elastic or ribbon should never be used as a way to keep the shoe on. Your shoe should fit correctly before you sew on the extras.