If the Blackberry is AT&T as well, then just put in your SIM card, and you should be good to go. You may need to upgrade your plan though (for data usage if you want to use the internet without being charged an insane amount of money).
I know for verizon and at&t it's 250 sent and received, so it would have to be 125 sent and 125 received, and I'm pretty sure that's how other companies do it as well