depends on YOU. your style should mirror who you are, because you have to be comfortable in what you're wearing in order to look sexy. but pinterest is a great place to look! http://www.pinterest.com/all/mens_fashion/ If you live in the states try H&M, they're the trendiest affordable place, Aldo is Gods gift to man for shoes. Tillys and Zumiez online has good stuff if you're more "punk / hipster". really though just wear what you find attractive.
I like boys with athletic shorts or long jeans. Button down or plaid shirts also. And let me think... Sneakers, converse high tops preferable. I LOVE HIGH TOPS ON BOYS. And um... I don't know the name but the kind of socks that reaches up to the bottom of your knee caps. That's just in general though. Like, it still varies on the person. Long blue jeans look cool, but if you are a bit overweight or something than maybe you shouldn't try jeans on, instead I'm thinking shorts. Typical guy shorts not athletic shorts. HIGH TOP LOOKS GOOD ON EVERYTHING I SWEAR AND TRUST ME.
I love button downs they make any guy look attractive if they wear them right. I also like it when guys wear backward baseball caps (not snapbacks hate those) but a button down or cool t shirt or jersey and jeans or khakis or basketball shorts if you're going to work out. Also men in uniform or dressed up i still dont know why women love it we just do. Don't wear Ed hardy sequined shirts or flatbrims or ghetto stuff or tank tops outside of the gym or cargo shorts and dont wear a tshirt every single day. a lot of girls like vineyard vines but im not a fan. cool it with the pastel shorts those things are horrible