My battery light is on.

It all started yesterday morning when the battery light came on. I drove it to work and it was running just fine, but coming back from work it's when it all started. In the freeway suddenly my car started to jerk and it sound like the rpm changes and the lights started to blink inside of the vehicle and the headlights started to go out. So I change a new battery this morning as well as the cables, but still the battery light came on. I took it to autozone and they said that there computer stated that the alternator read around 12. I'm not sure if that is good or bad. I just don't trust other peoples opinion because some of them work at a place like auto parts and still don't know allot about vehicles. So i'm asking you your opinion what could be the problem. Please help.