When I was 12 my mom said that I was only allowed lip balms or lipsticks and then eyeliners. 7th grade I was allowed to get Jane eyeshadows, I loved Goldie Looks and the silver shade, and then I discovered Surfer Girl. So since 8th grade I've been allowed to do what I want with my makeup and now I love mark by Avon, or just Avon, because the colours are great, products are great, and I don't pay an arm and a leg for them...so I can afford to get more stuff!
However since 8th grade I've been using Lancome eyeshadows. I have quite a collection and I appreciate the quality because they are good for eyes that can't normally tolerate eyeshadow. Most of my makeup, however, is drugstore stuff although it's changing to Avon because I buy mainly from the Outlet catalogues where I can get more for my buck. It is good to get some designer pieces if you can afford it and you like the quality or the colours. For example once I started using OPI nail polishes and was able to do my nails properly, I stopped using anything other than OPI or Avon because the quality wasn't as good. For eyeliners I tend to go Avon because yeah, they're more expensive than Wet/Wild, but they work the best for me. And for great lipglosses you can't beat Revlon or Bonne Bell.
One thing I do though is when I do plays with younger kids, I teach them about makeup-what the products are, how to apply them, etc, so that when they do plays in the future they'll have some knowledge of that kind of stuff. When you are young though, it's not necessary to have a huge makeup trunk. A mini drawer set will do wonders and then a small cosmetic bag to carry the daily stuff.