If I do a hard reset when the phone boots back up will I still have all my sim card info? Will the phone automatically work with AT&T or will I have to do anything special?

Thanks

A hard reset does nothing to the SIM. It just reinstalls the ROM image and wipes out all the data on the phone. After the reset it should automatically reconfigure to use whatever SIM is installed.

Note that if you're trying to back out a bad ROM from a third party, a hard reset isn't going to fix that, though.