I wanna join

That depends on what phone and plan you choose.

Since you posted in prepaid, you go to att.com, select prepaid,  and select a compatible phone,  and then buy whichever service plan you want to purchase.  You pay in full for all, and AT&T will send you a SIM card and a phone.

Or you can bring a compatible phone that you have purchased elsewhere and just purchase a service plan and they will send you a SIM card.

Prepaid service is like purchasing from a vending machine,  you pay for service and service is dispensed for 30 days.