TheNewGirl and Miss Crayon's got some awesome advice.
But ultimately, the less truly is more. The most important thing that changes your whole appearance is your eyebrows, go get them professionally groomed. Your eyebrows will change your WHOLE look as well as your hairstyle.
Get a hairstyle to match your face. Get a good conditioning mask on your hair, your natural hair colour is best. Get a good night's rest, swipe on some mascara and a nice gloss and go in there with confidence and a smile. A CFO of an international company once gave me the best advice when I was starting my first job overseas in a large corporation, he told me the best first impression isn't about how good you look, it's about how groomed and put together you look. Smile, confidence, something suitable for your age, and just look...fresh and clean. Not fake and uncomfortable. If you are in clothes and makeup that make you uncomfortable, you will show it. So just be comfortable
Start figuring out a skincare routine that works for YOU. Do some research, asks some professionals. When I was your age my friends and I were all about slathering makeup on, not caring about my diet or skincare and now as we're older, we live with acne scars, sun spots, blemishes, and regret not caring for our skin, rather then throwing things on it.
Mascara I like Lash Blast it's like under $10 at any drug store. And gloss...I really like the Sephora brand gloss. It's a little more than what you normally pay for at drugstore price but it's something you wear often so it's a good investment, think it's $12 for a gloss?
Get a really good makeup remover. The pigments from your makeup will seep into your pores and you'll be dealing with it if you don't remove all your makeup. Try makeupalley.com they have great reviews on useful, cheap drugstore products. But as TheNewGirl said, if it comes to your skin, go for a higher quality, might as well bring a lunch to school everyday and use the money on your skin. Trust us, when you get to a certain age (sooner than you think)...it will not cooperate with you, it'll wrinkle adn suddenly the many years out tanning will come back to haunt you.