So I am currently a graduate student. I am a socially awkward, shy nerd, but I want to dress more fashionably so I can be more attractive to girls (I know I have to get over the shyness also).
I usually wear t-shirts, polo shirts (including a couple Lacoste long-sleeve polos), jeans, hoodies, sweatshirts, and Nike shoes. Would I seem like a tool if I started wearing clothes from Banana republic, abercrombie, hollister, brooks brothers, etc?
I think you can sure get more fashionable without losing the core essence of what defines you. Geeky is in, and you might want to add the intelligent elements of nerdiness to your look! Add a touch of class and lose the hoodies/sweatshirts for sure. Keep it layered with a light blazer / cardigan instead, with sleeves rolled up. If you do want to wear a sweater opt for those geeky argyle ones. Try on a pair of stylish thick-framed glasses (zero power if your eyes are okay!). It's okay to go with sports shoes, but try out smarter ones, like ankle-length versions for example.
Of course the most important thing is to be comfortable and have loads of fun while trying your new look. If you feel great about yourself, it shows!
Like StarsGirl said just get clothes that make you feel comfortable because you can tell when someone feels uneasy...also if you're going for a new look maybe try a different haircut/hair style (that would probably attract more attention)
Google a website called artofmanliness. It's a great site with a ton of articles on all sorts of things that should help you get started with redefining your look. I've been going through a period of 'redefining' and I must say I have referred to that site plenty of times during such.
Well, the first thing I would do is hit the gym. Start working out and eating right. Get some muscle.
If you are looking for stylish clothes, stick with the more conservative things to start out. The most important thing is that they fit right. Ask the sales reps for advice. If you look in the mirror and think to yourself, "This just isn't me," then don't buy it.
Most importantly, maintain very good hygiene, and smile more often. Be nice to everyone you meet, and engage in small talk.