Ihave been bleeding for over fifteen days dark brown blood a little a day bt everyday, what might be the problem and what should ido?

Depends. If you are on the pill you may be adjusting. If not on the pill it may be the answer, go see your doc, at your age it could be an infection of the cervix but you likely aren't ovulating regularly thus not having the hormonal changes needed for a monthly cycle; consider a pregnancy test as well if applicible but go see your doc.