Also, I'm sure you're probably looking for advice on what type of clothes to get, or what pattern, colours, styles, etc. I'm going to give you some different advice. This will apply to you because you're probably a pretty average size (not fat, not skinny), so it's easy for you to follow.
1.(a) It doesn't matter what you wear, because you'll always look average (or worse) if your shoes are ugly, and your hair is bad. Fix those things up first, and then work on your clothes. Otherwise, average is the best you're going to do.
(b) You don't need to buy "skater" shoes like Vans to look cool. Just don't wear runners all the time. Are you running track, or playing sports all the time? It doesn't work.
2. (a)Buy stuff that fits. Seriously. It doesn't matter how good a shirt looks sitting on the rack, or on a mannequin. If it doesn't fit your body, it'll look like s***. I believe that any shirt that fits you well will look nicer than a "nice" shirt that fits a little too big. The stuff you already own may meet that criteria. It doesn't matter how ridiculous
(b) When you try on a shirt, the most important thing is whether the shoulder-to-shoulder width is correct, not how long it is. If it's too long, then of course there's a problem. Make sure the shoulders on the shirt (where the sleeves begin) and your shoulders meet. Otherwise it looks sloppy. Even an average looking shirt can look good if it fits really well. If you grow taller next year, shoulder width won't change too much.
3. You don't need more than 2 pairs of jeans, and you certainly don't need more than 3 pairs.
4. Sand, green, and brown coloured cargo pants won't get you noticed, and won't make you look cool. They're the most average you can get. If you want to look average, then own a pair. I try to avoid them now, but that's because I don't want to spend money just to blend in with the wallpaper. However, owning a pair isn't a sin, and it can look good. But you probably own a pair already. 😉
5. Don't forget your shoes and your hair!