Manually choose a plan for contracted Iphone?

Can I manually choose a plan later If I order a contracted iphone from Apple site or does it comes with a fixed plan already?
And how does contracted phone work in the US anyway? I have always been using contract free phone in my country.

You'll have to buy an unlocked iPhone to purchase without a fixed plan. They are full price so roughly $650. You can then take the unlocked phone to any carrier you wish later for the plan you choose. Contract phones can be bought from the Applestore for much cheaper but Apple will activate it to your carrier right there on a plan offered by the carrier. Best luck.