Though you won't get to any of them if you don't at least attempt to talk with her about boys, men, sex, drugs, friends, relationships and all the other icky topics you've probably been avoiding with a 100 yardstick.
If you are to do that, the most important advice I can give you, is to start talking about these subjects with others.
Next, learn to open up about your own experiences when you were her age.
Were you the teen girl who secretly had relations with older guys or men?
The bigger the age difference between your daughter and the guy she's communicating with, and the more breaths you'll probably want to take.
Feel free to grab a paper bag and breath into it if it helps you come back down to reality.
Allow your mind to consider what options you've got in your life in terms of where you live, where you work, what your philosophy about life is and don't forget to rehash your parental attitudes and education.
Wallowing in self-blame won't help either, and your daughter needs to work on her communication just as much.
Just be open, don't point fingers and realize that at this point, if you have sucky mother-daughter or father-daughter communication, and you need to start over.
You can tell that it's natural, because your not the only one going through this.
You don't have the only daughter texting, chatting with or sexting with guys or older men, nor will your daughter be the last.