Hair has a very, very wide range of styles, colors, and cuts for everyone to express themselves in a different way. Technically, nothing is "wrong" if you are expressing yourself, but never catch yourself dead doing these things wrong:
1. Over teasing/Creating a rat's nest - seriously, do you want to look like that? I'm sure everyone knows this one.
2. Learn your shampoo schedule, but please don't go in public with oily hair...seriously, if shampooing everyday isn't your thing then make sure it doesn't look greasy or find a style to disguise oily roots.
3. Dye your hair a completely opposite color without plans to keep up on it - if you have blonde hair, don't dye it black without intentions to keep dying the roots. If you bleach dark hair, keep up on it. Honey, it's not ombre to leave it, it's a straight growth line. Do the lovely ombres some justice.
4. And please please don't go in public with completely fried hair thinking it looks great. If your hair would catch fire easier than hay, please stop and take care of it :)