I tend to prefer the laid-back, yet sophisticated look... ie)nice jeans or kahki's, a nice shirt (can be long sleeve thin shirt), and lace-less slip-on leather shoes (polished black with the shine! - women love shoes lol). A simple yet affective way of dressing-up without going all the way is to wear kahki's with a nice blue dress-shirt (tucked in)and a white shirt underneath with the top 3 buttons undone. Golf shirts work well, and collared t-shirts. Just avoid the whole "preppy" look by NOT wearing Polo sweater with a dress-shirt underneath
I'm not a big-fan of Hawaiian shirts either, especially if they're the wrong colors. Make sure to wear clothes that fit your body... none of this buying clothes that are 3 sizes to big, it looks retarded imo.
As for appearance : hair tends to look its best when it's short. Shave if ya have to, get a good sun-tan, and wear a good cologne. I like Armani's Aqua di Gio, so that's what I use. Anything else is optional... ie) sun-glasses, watch, gold/silver chain, etc.
High-School shouldn't be a fashion show, but let's be honest... the better you dress, the better you feel. (ie : being more confident). Choose a good style, and change it around a little for some variety. Oh, and another item to avoid - soccer jersey's... I can understand wearing them if you're actually playing, otherwise they're just down right TACKY.