Go to a big department story like Macys or Nordstroms. They have makeup counters from the different designers and they hire people to suggest what makeup colors you should wear and usually if you have time to spend they'll sit down and actually do your makeup for you with the right colors. You don't have to buy anything but I suggest you do because you'll learn how to do makeup more and be able to change your look. Yeah usually the makeup is more expensive at department stores rather than like CVS but its definitely worth it. Since you're still young, if you want to go for a more edgy look than if your mall has a MAC store I'd definitely suggest that over department stores. Your makeup will change a lot about how you look.
As for your hair, go to your hairstylist and ask them what they suggest you do. Go for things you don't normally get. Even like a hair color change would be good.
You don't really need to know fashion to wear good clothes. You just need to know what fits and looks good on you. Go to a store and try on a lot of different things, especially things you wouldn't try on. Look in the mirror and then look from your neck down. Ask yourself if you saw this outfit on someone what would you think? Completely pretend like you're not wearing it and see how your reaction is.
You don't have to do anything drastic to change your look.
I always kept my hair long with some layers in high school. When I graduated, I cut it shorter with tons of layers and it made a huge difference. And now, I grew my hair out again and got sideswept bangs. Bangs are actually a really good way to update your look, if you're into the idea of having them. They also give you more options for hairstyles since you can pin them back in different ways.
For your clothes--find new ways to wear them instead of buying a whole new wardrobe. My style was really simple when I was younger, but I've realized that layering and adding some accessories is a way to change up your look. You also have more options that way. Buy a few new pieces that you can wear with multiple outfits.
I can't really help you with makeup since I've never worn it (but that just goes to show that it's not essential to getting a makeover). If I started to wear it though, I would probably go to my mom or female cousins for tips. They've been wearing it for years, so they know the "rules."
You should try to sign up for that TLC show "What Not To Wear." Chances you'll get on there are slim, but what the hell, maybe. I also love doing that kind of stuff so if you want to send me photos and I can give you suggestions I would be glad to help you out!
Recently, For my sense of style, I found a celebrity or a person I have seen or know that I think has an awesome fashion sense. I then write down the things that I like about that particular style, how the clothes fit, what kinds of colors are in the clothes, etc. When I go shopping I BRING it with me. It's significantly helped my fashion sense. I really like Vanessa Hudgens sense of style and attempted to bring some of that into my own.
Yeh I can understand that is frustrating when people say that (thankfully I've changed loads since high school but that's because I needed to because I looked a mess lol) I don't know where you are but in the city near where I live they have beauty salons that offer makeovers (hair and makeup) and just go shopping with a friend and go for a new look (I recently dyed my hair blonde) and I'm still getting used to it because I look so diff so if you want to do something drastic maybe change your hair colour.