If buy an iPhone 5 at Apple Store

Can I also switch to my own contract? I am on my parents plan as of right now, and need to go to my own contract. If I get an iPhone 5 at an Apple Store, can I switch my contract to my own at the Apple Store? Would they be able to do that for me? Or is that something that only Verizon can do? I think it wouldn't work this way, but I just want the iPhone 5 already, and cannot wait!

Or could I go to a Radioshack or Walmart or Target to buy an iPhone 5 and have them switch to my contract? Again, high hopes, but just asking.

Thank you in advance.