How do I apply my att. Prepaid plan to my new phone when I'm not sure if there is a sim card b/c in about phone there is no phone number.

As noted above, you move the SIM card from your old phone. If you don’t have a SIM card to move, you need to determine if the new phone came with a SIM card. If not, get a new one. Then you need to go to an ATT store or call to have the SIM card activated with your number. Do not activate online, if you do you’ll get a new number.

If you are currently an AT&T customer, you existing phone has a Sim card.  All At&t phones have sim cards, and always had sim cards.

If you don't know how to access your sim card to move it to the new phone, please post the full make and model and any other information about your current phone so that we may direct you on how to access the SIM card.  If you have a previous prepaid phone, it very likely has a removable plastic back, and the SIM card is inside the back.