How should i break up with my girlfriend?

i kno im 15 n maybe to young n all but my girlfriend is startin to turn me off. all she wants to talk about is n she always wants me to turn her on. so often that its annoying. (ive never had with her tho) she also lives in illinios nd i live in missouri. another thing is she likes to flirt with other guys, nd girls (she told me later in the relationship that she was "bi" nd i played like it was cool) i had mad feelings for her when we first met bout 2 months ago nd im thinking of breaking up with her but ive never broken up with anyone. i dont want her to get mad. oh nd one more thing, she went with 2 of my buddies but thier kool. can someone give me advice?