If you could post pics, perhaps I could give my personal recommendations regarding size, however, do not go above a midsize B for certain.
Placing the implant submuscularly (behind the muscle) would be preferred in your situation. Less risk for capsular contracture, although it is a longer operation.
Most people opt for round shaped implants, however, a contoured implant (also known as teardrop shaped) looks more natural, especially on a small girl, such as yourself. The profile of your breasts (protrusion of upper part of cleavage) will not just out as much as well, thusly creating a believable appearance.