I can't add anything more to what San said. She's listed all the major hot shopping spots in Melbourne.
One piece of advice: If you're on a budget the further south down Chapel Street you go, the cheaper many of the shops get.
Toorak = Insanely pricey
South Yarra = pricey
Prahran = Somewhat affordable
Windsor = Lots of op shops!
I live about five minutes from Chapel Street (arse-end in St Kilda), but sadly as a student I can rarely enjoy its shoppingly pleasures.