Others have some great advice; here’s my two cents.
It takes a little work to change your style and look pulled together. Don’t expect it to come naturally or easily. I’d start by paying attention to what other people are wearing. Take note of styles and looks you like and what pieces you need to attain that look. I don’t think fashion magazines are very useful for this; it can be hard to interpret runway looks donw to everyday life and too often, the magazines push what the retailers want them to sell, rather than what’s really current.
You also need to find stores that carry the kind of clothes you want to wear. If you don’t like shopping, it’s a lot easier nowdays 'cause you can check out stores online and see what they carry and what there prices are. And the cheaper stores have gotten much better at staying current. Old Navy and Target usually have good selections of stylish clothes.
As far as fit goes, if you have cable, check out the BBC version of “What Not to Wear”. The American version isn’t bad (tho’ they stick every one in pointy toes and jackets which isn’t always practical or everyone’s cup o’tea) but the BBC version is much better at picking out clothes that really flatter a specific body type. Suzanne (or Trinny, I always get them confused), one of the BBC WNTW hosts, has a body type very similar to yours and will have good suggestions.
There are also books from both shows, as well as other books on how to pick clothing I don’t know if it’s worth buying the books 'cause they’re usually not things you reread a lot but you could go down to Borders and spend a few hours browsing. One thing all the experts advise is making sure you wear a good bra. I used to wear those stretchy pull-over bras until my friends finally shamed me into getting a real bra. I was amazed how much better (and thinner) it made me look.
Make-up takes practice. For everyday at school, you really don’t need more than some mascara and gloss. For evening, get one of those cheap drugstore pallettes and play with it. There are sites online about make-up application and there are also some good books (usually right next to the fashion section at Borders). I like to play with make-up right before bed. I can take risks since I’m just about to wash my face anyway.
I think it’s really hard to make long, thin hair look good. If you get it cut to shoulder length, it will get a little bounce from hitting your shoulders and look fuller and blowdrying also makes hair look fuller and shinier. (and blowdring can be easy, all I ever do is blast it upside down and then do a quick curl under for the ends and I’m done. Nothing fancy at all.) I second alice_in_wonderland’s suggestion of a good haircut. You should at least have an idea of the type of style you want before you go. Magazines are good for this. You won’t look exactly like the picture, of course, but you’ll be able to give the stylist the gist of what you want.
I used to think I had really greasy hair and I blasted it with “oily hair” shampoos. I’ve switched to a “normal” shampoo and my hair is significatly less greasy. I think over drying caused a oil overproduction rebound. So you might experiment a little with shampoos. Don’t think your hair has to feel really sqeaky when yo wask it, the squeak means it’s probably over dry.
Wow, I wrote way more than I originally intended. Anway, my basic message is this: clothes and make-up should be fun, don’t be afraid to experiment. But, on the other hand, just like any other hobby or skill, don’t expect to just magically pick it up in one night, it’ll take a little thought and practice. 
LY/MI