it depends of you want ot be more like blair or serena, thier stlye is quite different.
If blair, try floral, or something at urbanoutfitters.com favourite blair dress:http://i42.tinypic.com/2egh36r.jpg
of serena, try solids with a detail design (like detailed straps) favourite serena dress: http://www.polyvore.com/cgi/img-set/BQcDAAAAAwoDanBnAAAABC5vdXQKFlZEZjdldW5LM2hHWGpkdzk3eDNlY2cAAAACaWQKAXgAAAAEc2l6ZQ.jpg
I hope this puts you in the right direction! have fun!
p.s....love gossip girl!!!!!