Buying unlock iphone 5 from apple?

I'm 15 and I'm saving up to buy an unlock iphone 5 from apple. I want it unlock because I don't want a contract. I want to know a cheap prepaid plan that works for the iphone 5. I was thinking about straight Talk, but I saw your can't send mms and I don't want to do the tmobile sim swap. My other choice is tmobile simple choice plan for 50 per month. But I need more information on the tmobile plan, can I send/get mms? Do I haveto cut the Simpsons card to nano? Is it truly contract free? What if I can't pay for a month? Will they damaged my mom's credit score?(I'm giving my mom the money since I don't have a credit card) thanks in advance.

I don't know much about t mobile but you should try virgin mobile. They have iPhones on there network and if you don't talk much there's a 35 a month plan with unlimited internet and texting with 300 talk minutes. As for t mobile, if it says prepaid then there's no contract just like any other prepaid network. If you were to go with tmobile and didn't pay you can still reactivate service with the same number.