If you like to stay in the city center walk or take the U2 to Gänsemarkt. Here you'll find the usual suspects as American Apparel, Gucci, Replay and Levi's as well as small yet brilliant shops such as Tate (good for sneakers and street wear), Thomas I-Punkt (great for more grown up fashion) and Jun (just like Tate only not that limited).
If you'd like to stroll around take the U2 to Messehallen and immerge into the small shops of Markstrasse. You'll find both designer and street wear fashion hear as well as a great retro shop called "Hot Dogs". Then take a walk over the Schanzenmarkt (a flea market, every Saturday) and go to the Schulterblatt for a coffee and toast. If you have some time left walk down Susannenstrasse, there are a few nice shops as well.