As Tmiles said, a lot of it is just having it fit well, and wearing it with confidence. And also, probably one of those most difficult for a lot of us young or old, is caring less about what other people think in general of our looks. Sure, friends and family can matter to a degree, but other people?
Biggest piece of advice, fashion or otherwise, that I wish I had managed to embrace a couple of decades ago was care and think more about what YOU feel and desire, and a whole lot less about what society/others/industry/media/etc says you should care and think about. Sure if you see something you think you'd look good in give it a try. But don't try to emulate a particular look or behavior just because it's "cool" or the in thing.
Do that, and YOU will be setting fashions and behaviors for those around you. Just don't accept harsh words from those trying to tear you down, instead of honest and criticisms to try and help you build up.