Advise please

Any advise....I adopted my friends teenager a 15 year old, she has been talking to men. (By men I mean adults.) She has been taking exposed pictures of herself and talking very bad to them.so I took her phone, I found out she is hiding a friend's phone. I found out because my 3 year old took it out of her backpack and told me, and I thought I saw it but the teen hid it so I was giving her the benefit of the doubt, well one of her friends confirmed that she was using a phone as well. It's not the fact she's using the phone but hiding it from me and lying, it makes me worried about what else she is doing, I want her to know I care but it needs to stop. Thank you for your help.