1. Make sure it is unlocked.
How do I determine if my BlackBerry is "unlocked"? - BlackBerryFAQ
How do I unlock my BlackBerry? - BlackBerryFAQ
2. Call ATT, get a voice account and add the BlackBerry Data Plan it.
3. Activate the device on ATT. You'll need the PIN and IMEI of the device (found at Options > Status or on the white sticker under the battery.