Please please please make sure that your concealer matches your skin. The best place to test is on your cheek going down and to your jaw. Nothing looks more ridiculous then anyone who wears makeup that isnt the right colour, and you can see it on their jaw.
Get a really good eye makeup remover. Nivea is really good...both the the liquid and the cream. That way you can play around with your makeup and it'll be easy to remove without scrubbing.
If you can, go to a store that has a makeup counter and ask questions. They will let you know what colours will work for you best. Don't feel pressured to buy makeup for them, as for a teenager...it is overpriced. Covergirl, L'Oreal, Maybelline and Revlon are all excellent brands to just start out with.
Since you aren't sure of what your style is right now....try buying clothes at outlet stores if possible as they will be less expensive....the store is just trying to move the clothes. It will make it a lot easier to get a different variety of clothes for less, letting you play around with what you think you like. Pick up things that you wouldnt normally. Try EVERYTHING on that catches your eye.
Guess what? Lots of adults don't have their lives together either! As a teenager, you are still discovering who you are, so don't feel like you should know already. It takes time, mistakes, learning and patience.
I always found when I am not happy on the inside, I am not happy with whats on the outside either. So possibly also try and look inside yourself and see what is going on in there. It will affect your outlook.
PS...Adults have access to the teen section and we are allowed to post on a topics that we feel could use an adult opinion on.