For my homework we have to write a letter to agony aunt. and then answer them as if you were the agony aunt.
mine is that my best friend breasts hurt and she is worrying.
what can i say for the agony aunt

Say exactly what you're thinking. Just let the words flow. Something like,
Dear agony aunt,
My best friend's breasts hurt and it's worrying her a lot. I don't know how to help her, or if I even can.
signed x
And then your agony aunt part can say something along the lines of,
I'm sorry to hear that your friend's breasts hurt and it's worrying her. It could be something as bad as cancer, or something as simple as her breasts growing. Breasts sometimes hurt for no reason at all, but if it's persisting, I would probably go see a doctor. Suggest that gently, so as to not worry her more. They can run all sorts of tests to make sure she's okay. And she can look up self-breast exams online, they're very simple and take barely any time at all. Young women need to take care of themselves and to know what's going on in your body is one way to make sure you stay healthy for a long time. As her best friend, I'd suggest you be sure to support her in her going to the doctor, it can be a bit frightening, considering how little they teach people today about their bodies. Try to talk to her about it, and be as encouraging as you can. She's not going to die, she's most likely going to be fine. It'll be alright, dear. Take care!