I'm a jeans and t-shirt girl. Of course it helps that I work at a game company testing software and that's the standard dress code for the place.

I've never really bothered with makeup and perfume just makes me sneeze so can't help you there. If you're comfortable in jeans and t-shirt, pick up a really nice pair of jeans and some cute tops. That will go far in changing the look. I've also found that boots or low heels are good, too.