T-Mobile typically ships registered SIM cards when you sign up for a subscription plan. However, if you're a prepaid subscriber ordering online or by phone, you're out of luck on the automatic registration. However, it is still possible to register your SIM card for your T-Mobile-purchased phone, or an unlocked phone purchased elsewhere.
Access T-Mobile's SIM card registration website. (See "Resources.")
Enter the information requested on the website, such as your name, number, SIM card number and prepaid plan.
Click the "Activate Phone" button to register the SIM card.
Insert the SIM card into your T-Mobile phone. The slot for the card is usually located in the battery compartment of the phone.