Where do i go from here with a grown *** 15 yr old?

Well im a mother of 2 boys and also take care of my sister kids boy &a girl Well the nephew he listen and some like Well behaved Well for the niece she thinks she know every thing .Well my niece is15 yrs old and yesterday she came to me ask can she go Jang out with her girlfriend i told her Yes Well When she came back home it was something different about here like she had a glow going on about her but i didnt say anything because i didnt want to humo to any conclusi�n so im was going to bed and i pick up the house phone to Tell her to not go back out side When i over here her conversation on the phone that she had sex so her friend ask her Did she *** and she says i dont know because a girl cannt Tell if she *** boy was i mad so she ask him about His self-confidence and he says that im a hustler and that he drop out of school now im not sitting Well with none of this non since at all so she cursing like a grown-up which that didnt sit Well with me nether .in my book my kids can talk to me about anything and they have done it in the pass .im upset that she having sex at such a young.age messing with a good for nothing thug .now i always ask my niece was she a virgin and she swore to me that she was and now that i found out that she not .i cannt trust her .all my kids know that they can talk to me about anything .and i mean anything but i just dont understand Why she lied to me this time how many othere times she Did this .and by the way the boy is 20 .now what should i do i want to take her to get birthcontol .but i dont want it to b like its ok to spead her legs Open at any giving time she only 15 there is more boys to come she going to like many more .what do i say Or do.her father is not in her life right now.so i dont want her running to a man for love that she missing please help