How can I dress maturely without looking awkward?

I'm gonna be 21 this year, i'm 5ft 11in and weigh 123 pounds. I need some style help, my style is basically Aeropostale t-shirts in size XS and slim fitting jeans. I've looked around for more "adult male" type clothing but they are all big on me and make me look even younger (and skinny awkward) because they're baggy. What should I do to look mature and have style?