You don't give much detail. So try the gym if you need to lose a little, dress a little younger or even go to one of those makeup stores at your mall for some new or different ideas. Hairstyle may be another factor. I like long hair no shorter then touching the shoulders.
Sometimes that's about all we can do with what mother nature gave us.
Exercise, limit calorie intake and eat healthier foods as much as you can, go to a good hair salon and let them help you decide on a better hairdo and/or color -- spending the money is worth it if its a good place. A BIG thing is how you dress...that alone can make you look older. Go to the mall someday and just watch girls your age or maybe a little younger and see how they dress -- get ideas, write them down and then go shopping. Also, buying a few good fashion magazines and looking through them for ideas, products, etc has helped me. It can be done! (Oh, and I agree that longer hair -- or at least shoulder length is usually better although some women look great with short hair).
Well I would start with making sure you are fit. A great body always makes you look younger. Also go to a good hair stylist and let them tell you what would look good with your coloring and face shape. Also dressing more trendy helps.