Choose any of the available no contract plans. For best value, start with the cheapest plan that you think may fit your needs. You can always go up to a different plan if you need to.
Choose the new phone you want, and then select the length of the installment plan to pay it off. Terms include 12, 24 and 36 months.
No lock-in contracts let you move to different plans as you needs change, or as hot new deals come along. Never feel stuck in your plan again.