How can I find out how to unlock my AT&T cell phone?

I have the samsung blackjack and my provider is AT&T. I am sick of paying $140 a month plus i have so many issues with them. I have found so many other providers that are much much cheaper and i will get what i have now. unlimited text talk and data. I was looking on walmart.com and they have straight talk it is prepaid. for $45 i can get 1000 anytime minutes and unlimited text and data which is perfect for me. but i would have to buy a phone. is there a way i can unlock the phone i have now to work with that?