How should I go about buying a new phone?

[Alex]

So, a bit of background- I'm a male high school student in Sydney, NSW and am looking into buying a new phone, and a new plan. I also need a new iPod, so I was thinking of getting an iPhone 4s, like a 2in1 kind of thing. I'm currently with Vodafone now, with a crappy pre-paid deal, because my parents thought I would be using credit at a extremely low rate. I was hoping to keep my number, but my real question is, do I buy the phone through a plan or outright and pick up a seperate, probably cheaper plan? And would I be able to use any carrier or can it only be Vodafone, Optus and Telstra. Any answers are appreciated.