How should I buy an iPhone 5s?

[AintItFun?]

If you buy the iPhone unlocked, do it through T-Mobile. They have the cheapest service and they offer truly unlimited data for $50 a month (and since you're on a prepaid plan it's exactly that with no taxes!). The service can be a little spotty, but that's true of most mobile services when you don't live in the city or near its core...at any rate, if you choose Straight Talk know that while it only costs $45 a month there is slower internet and data caps. Coming from a dumb phone Straight talk should be fine, but if you're going to be on your Twitter, Netflix, Instagram, and other services (which is bound to happen if you own a smartphone) just know your phone is going to eat up the 100mb they give you per day.

EDIT: I have no idea what Hugo is saying, iPhones have sim cards. They're on the left side near the bottom of the phone on the bezel. It takes a little wedging of the finger, but you can pop it open.