First international flight, I have a few questions?

If all your flights are with the same airline or a partner of that airline then it will be sent all the way to Cairo without having to check it in. You can ask them at atlanta when you check-in but I'm pretty sure they'll go all the way through. Yes, if you booked all your flights at the same time you will get boarding pes for all your flights at atlanta. With the layover situation. Try walking around. There's probably lots of shops, also there might be a computer room where you can surf the net (they're usually not free though) or you can go to some restaurants or cafes there at the airport. Also keep yourself smelling clean too because of the longtime between showers and stuff like that. I've experienced a few big layovers and that's what I did to p the time.