I just recived my new phone in the mail today. I need to know hiw do i register my existing phone number to the new phone and sim card. As this sim card is a different size than the one in my old phone.

I can do that online for my account. Log onto your account. Click on "Report a lost or stolen device" (just bear with me here). Then you should see an option to "Change SIM". Click that and enter the new SIM number.

This works for my postpaid account. If you're prepaid, it may be different.

Double check your current SIM to make sure that there's not a smaller punch-out center section. The easiest thing to do is just swap the physical SIM.