Just walked in on my daughter......

I just went out to get groceries and told my 13 year old daughter that I'd be back In a little over an hour. I got to the grocery store and it was closed so I went back home. When I walked in the house, I saw shoes at the door that I've never seen before. So I went to my daughters room to see if she had a friend over. I opened the door and there she was having sex with some guy on her bed!! Somebody give me some advise on what to say to her !!