How do new iphone plans work?

If I want to get a new Iphone 7, I know I will have to pay for a monthly phone bill, but do I also have to pay for the phone itself monthly? Can I pay the phone price itself in cash?

I heard something about being able to upgrade to the newest iphone for free when they come out too, how does this work?

Added (1). Also I will be at basic training and unable to use my phone for 3 months, do i still have to pay for my phone bill then?

BUY IPHONE 7 WITH CASH ONE SHOOT, THEN YOU ONLY PAY FOR THE Wi-Fi CHARGE MONTHLY ON WHATEVER YOU USE THE TOTAL Wi-Fi ON LINE TIME.