Do you mean you need a SIM card for AT&T service? Or you brough a used phone that do not take any SIM card (no SIM card slot)?? Phone without SIM card slot are mostly run on CDMA,it cannot be use for AT&T service.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.