[Stark Naked]

It's not so much about what you wear, but how you wear it. Like Suits pointed out, the fit of your clothes are very important, even moreso than what the clothes themselves are. I've seen plenty of guys in ill-fitting suits and baggy polos, and at the same time guys in jeans, t-shirts and sneakers that still look sharply dressed. Take into consideration your body type and personal style, and work from there. As you said, you're a t-shirt and jeans kind of guy, which can work, as well as you have jeans with a good flattering cut, and a shirt that hugs your frame in the right places. While jeans might not be "in" right now they are a classic staple and can look great when worn well. Try out some collared shirts with them - button downs, polos, long sleeve, short sleeve, etc. - and find ones that look good on you. Do some homework, hit up a thrift store, and it won't take you too long to go from slob to style while dressing in pretty much the same manner you did before.