Ladies, I need some advice.

I found my boyfriends daughter with my thongs!! And they were all crusty up with her juices. I'm so grossed out. Aside from that, I'm upset that she would steal my thongs and wear them! I spoke to my boyfriend and he said I need to tell her something. He said I should confront her and tell her to stop getting my stuff. I have also found her with my bras. I don't know how to tell her. Only reason why her dad wants ME to tell her is so she can respect me!! So I'm here to ask, how can I confront her and tell her to stop getting my stuff? It's gross and she's only 15, she shouldn't be wearing thongs. I'm kind of nervous. Please help. She also has an attitude so I need to be as calm as I can. 
UPDATE: 
I spoke to her, we went in her room and I showed her the thongs. She came me some bs about how it could be her sisters (which hasn't came over for at least 9 months) or they could be my boyfriends ex's thongs. I told her I knew what was mine and if it was her or not I was still going to speak up. I told her first of all it was gross because I've used those before. Second, it's disrespectful because those belong to me and if she ever wants my stuff or something similar, we can go shopping and I'll buy her that. She told me her mom does that for her. So I said ok well I don't ever want to see my stuff in your room, I said I wasn't upset at her but next time I simply asked for her to ask for something. I also told her I haven't told her dad, which is a lie, but if I did catch her again with my stuff, I would tell him something. She just said okay. I said thank you and walked away.