http://www.girlshealth.gov/ was helpful to me. It's designed for adolescent and teen girls, and it helped me as a parent think through what I might want to talk about with my daughter. They cover sorts of topics, health, nutrition, relationships (with a section on safe dating). It's a lot of fun and I think most young girls would like this site. They even have fiction written by girls.
After I spent some time there, I had my daughter visit the site with me. She was impressed I knew which tabs to click to find things!