I've been put in charge of taking the girls' night at a youth group and I am doing it on decision making, relationships, sex, etc. I would love it if you could give me some tips on what to say to them about it. Their ages are between 13 and 17. I was thinking about what might be appropriate for the younger ones. Any suggestions will be great.
What works best for any group is material that they can put into practice either immediately or in the near future. Applied learning stays with a person for a long time. "Therefore lay aside all filthiness and overflow of wickedness, and receive with meekness the implanted word, which is able to save your souls. But be doers of the word, and not hearers only, deceiving yourselves. For if anyone is a hearer of the word and not a doer, he is like a man observing his natural face in a mirror; for he observes himself, goes away, and immediately forgets what kind of man he was. But he who looks into the perfect law of liberty and continues in it, and is not a forgetful hearer but a doer of the work, this one will be blessed in what he does" (James 1:21-25).
What also works well is to have a story to hang your memories upon. You see this constantly in Jesus' teachings. He used illustrations and short stories that help people recall the principles he was teaching them.
There is a series of such lessons aimed just at the age group you want to teach called Moral Principles for Young People on this web site. Each lesson starts with a story and includes a set of starter discussion questions. Each also includes a long list of verses for the teacher to study in advance. What I would suggest is reading the verses and jotting notes beside them concerning what the passage talks about. As the discussion hits on particular issues, you will then be able to direct the group to the appropriate passage that might address the issue. The lessons are designed to be presented in any order, so pick the ones you think will best suit your group.
For the more specific topic of sex, see Growing Up in the Lord: A Study for Teenage Girls.