Normally, you can call ATT and ask them for the SIM unlock code for your phone, if you have been a post-paid account holder in good standing for over 90 days.
HOWEVER, there was a new legal settlement earlier this year, that says ATT does not need to supply the SIM unlock code for any new "exclusive" ATT phones for 10 months after the phone's release. Since the Aria was released in June, you would still need to wait another 4 months. But after that time, they legally need to give you the code.
Another option is to just go on eBay or search Google for paid SIM unlock services for your phone. These services will usually get you the code for $10-20. Try to pick a service with a good eBay rating, or otherwise good reputation or a referral.