If you're the parent of a young girl, then the first thing you should do is discuss her ideas. Although it's your house, it is her bedroom after all, and she still needs to feel comfortable in what is basically her haven. So if she likes a lot of cuddly toys, incorporate soft cushions into the final bedroom design. Work the color scheme around her favorite posters, whether it's of bands or celebrities, so that you have an all-encompassing mix of your choice of teenage girl's bedrooms. A good approach is to make the room fairly neutral, and then changes can be made along the way.