Need help

There are two issues here. First, the phone must be SIM-unlocked if it originally came from another carrier than AT&T. Second, it'll work on the 1900MHz frequency that AT&T uses, but you may experience large coverage gaps because AT&T uses both the 850 AND 1900MHz bands. Phones without the 850 band (and no, 900 isn't the same thing) won't work as well as a phone that natively supports both of AT&T's bands.