One of the questions that many women with smaller breast ask has to do with measuring properly for their petite bra size. The fact of the matter is, it doesn’t matter if you have smaller breasts or if you are large busted, you are going to follow the same basic procedure. You would measure around your upper torso, under your breasts to get the band size and then measure the distance around your torso at the largest part of the breasts to determine the cup size (Source: 36aa bra sizes at Lula Lu). There are charts available which will make this relatively easy.
It is also a good idea for you to choose a bra that not only fits according to your measurements but is comfortable. Some bra manufacturers do not specialize in smaller bra sizes, although they may make them and have them available at the store. It is always a better idea if you purchase your bra from a manufacturer that is known for producing a quality bra for women with smaller breasts.
Finally, look at yourself in the mirror and see if it fits you properly. You can also check for some of the common problems with bras, such as having the strap fall off of your shoulder or having it be so tight around the strap that it is going to dig into your skin. These are not problems with your size, they are problems with the size of the bra and the fact that you are likely using one that is not fitting you properly.