The first place to look for a missing teenage girl would be among her friends. It is often possible that a teenage girl would have friends (particularly male friends) that her parents might not approve of. If she has run away, she might well have gone to stay with one of them. This is especially true if she has had a boyfriend that her parents have forbidden her to see.
Another likely place to look for a missing teen would be in areas of cities where teen runaways tend to congregate. These areas may be around a university campus or a downtown. They may include homeless shelters or other places where the runaways could get something to eat or a place to sleep.
Sadly, it would also be important to look for a missing teenage girl in areas where prostitution is common. Teen runaways often end up having to resort to prostitution.