Here's one massive tip that will save you these silly questions, find a hot girl and tell her this same dilema. She will go shopping with you and tell you what to buy. She will pick clothes she finds atractive and in turn will probably be attracted to you for wearing them, need I say more?
However if your problem is that you don't feel secure chatting up the ladies in your scraggy atire, then there are quite a few options. I'm assuming you are British (just becasue you said Uni, instead of college) Unless you have a particular look you want just go to somewhere like Next or even Gap! most of the shops do the thinking for you, with all their posters and mannequins. Another tip, purely from looking at University challenge I am assuming that jumpers and collars are very in - perhaps go for that if you want to look like a Uni student.
Skater: Zumiez, PacsunSlim jeans. don't buy tight ones cause they suck. Graphic Tees, go with what you think looks best. make it unique. Remember, you're trying to get a look that says you're unique. and the reason I'm not wearing anything unique in my picture is cause it was a spirit day in my school that week and the colors were green yellow.
Prep: Abercrombie & Fitch, American Eagle, Hollister. Short Striped Tees, Slim-ish jeans. and accessorize, remember, your going for a beach guy sandy look.
Gangstah: Bosswear, Finishline. Baggy Jeans, preferably in a color that stands out. Remember: you're trying to get that look that says, IM GANGSTA, GET OUT MAH WAY!
and if you don't like what you see, mix it up. but don't just make it random. Skater+prep is a good combo, but never Gangsta+prep. too much contrast. Gangsta+Skater is also a good combo. skater goes good with almost everything. and Gangsta looks don't get you much chicks. if that's what you're hoping for. oh, and one last piece of advice; it takes time to find your style. don't rush it. SERIOUSLY. that is some serious advice right there.