Almost every 16 year old girl should be wearing a bra. Usually girls start wearing "training bras" as young as 11 or 12 years old. (This is the United States, at least.) I suggest that you talk to your mom about buying a few bras for yourself. Chances are, the one that you stole from your mom doesn't fit you correctly, not to mention that it's not very nice to steal! If you feel uncomfortable talking to your mother, that's understandable, but there is nothing to be ashamed of! After you buy your first couple of bras, you won't feel awkward about it anymore.
If you are a 16 year old guy, on the other hand ... I'm not sure what to tell you! That's a little more complicated.