How to most efficiently buy an iPhone 5?

Okay so the phone I have right now is a plan with Tmobile and I think its like a family plan with all our phoes, and I'm super unsmart at understanding technology to how would I switch phones with the same phone for the best price

Sell your phone and use that money for a down payment on the t mobile IPhones, 20 dollars a month for 2 yrs., including 150 for down payment