How do I activate a tmobile prepaid sim card?

I already have a tmobile phone, but its not prepaid. Well I saw that I can buy the sim card online. But how do I activate it if I didnt buy a phone. I saw that you have to submit some codes in order to activate it. But theyre from the tmobile prepaid box. What do I do?How do I activate a tmobile prepaid sim card?
it should let you activate with the phone you already have ?

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