Before attempting to activate your AT&T SIM card, check whether your phone or device is already active. To do this, install the SIM card into the device and turn it on.
Place an outgoing call on your device or alternatively, start a data session without using Wi-Fi. If the connection is successful, your device is active.
If you need to transfer your contacts or other content to a new device, visit AT&T's Copy and Transfer center before beginning the line activation. Visit the AT&T activation page, and select your activation type. Upon selecting your activation type, enter the wireless number you want to activate and your billing or shipping ZIP code. Select Continue, and then follow the on-screen prompts and instructions to complete your activation. In case you don't know how to insert the SIM card into your device, visit AT&T's Device How-To Center, select your device's brand and model, and enter "Insert SIM" in the search field to get instructions on inserting the card.
The next step after activating the SIM card is adding credit to your line. You can do this online or over the phone with AT&T. You can also use a third-party online service, such as Recharge.com.