Help i am confused

I am new to this i havea t-mobile i bought. the problem i have is where i am using it t-mobile service will not work i was told if i bought a cingular sim card it would work i got one and when i put it in it rells me sim card registration failed what do i do now.

check your sim card if that's activated! i you gonna ask me that sim is not activated. this is not the issue of cell phone lock to diff sim card. this sim card is not activated and give your service provider a ring.