I wanna change phone service into pre-paid phone?

i used T-mobile.

i wanna change its currently service into 'pre-paid' cuz i wanna use it like only 10days and i'm not using it that much !!

so, here's question, how much money will be take basically?

And how much money do i have to charge for for 10days?I wanna change phone service into pre-paid phone?
with tmobile they have prepaid cards.

you buy a card of different amounts and it gives you so many minutes. if you run out just go purchase another card and reload