Default [Q] newbie galaxy s3

For some reason I restarted my att s3 and its stuck on galaxy s3 page. I been reading alot of this stuff and i think its over my head. I bought the phone used and it has twrp version 2.5 on it. It say it has no OS on it. I wouldnt even mind it in stock form. I use straight talk.If anyone would want to walk me through this it would be greatly appreciated. I live in pa if anyone is nearby. Scared and dont want to ruin this phone.