What should I do to change my wardrobe?

I am a freshman in high school. My wardrobe at the moment is t shirts and jeans. I am petite, so a lot of my clothes are a little big. I look at some of the girls in my school, and most of them have cute outfits. I feel really frumpy in my clothes when i look at them. Like the school president is really pretty and she always looks good and professional. I want to look ready for success and I want to look good/cute. Please outfit advice ans whatever.