Hey, I'm a Junior in High School, so I've for sure been there, done that.
Really what you want to do, is express who you are through your clothes. If you like the skinny jeans and graphic tees, find some really cute, dark wash skinnies, and a not-to-tight graphic, and some really cute sneakers, or ballet flats. (Also, no matter what height you are, you can totally rock heels in skinnies.)
If you're into the boho-chic look, find a totally cute knee-length skirt (JCP has some REALLY cute ones) and a fitted, basic top.
If you wanna wear just a tee and some shorts, take a pair of old jeans that you never wear, and cut them off, or dye them. It's inexpensive, and a good way to freshen up your wardrobe. A cute, bold belt always looks good, especially with converse.
As for the makeup? If you're going to wear any, go natural. You're young, you don't need a lot. Some lip gloss and a little mascara will look natural, and like your not trying to hard. Hair? Straighten it, wave it, whatever you feel most comfortable in. Accessories? Don't go overboard, but have some cute statement pieces.
Hope it helps (:
Got any questions or want some pics for ideas? email me at shannonlynn3@gmail.com.