Looking to transfer our phones on a family plan.

Looking into transferring my Verizon prepaid accounts over to an AT&T family plan. I didn't see that within the options for plans, do I need to set up an account separately to begin with and then combine them? How do I gain access to sim cards for to bring your own phones and have them activated online? We always do automatic payments because that is easier, but I wasn't sure how to get everything started.