I have the AT&T Galaxy S3 on 4.1.2 and want to root for the first time. I've looked around and have seen that chainfire would be a good way for a beginner like me to use. I was going to do it, but then I read something that said the Bootloader had to be unlocked. I don't know what a bootloader is and if mine is unlocked. If it means if the phone is unlocked so I can use it on other carriers then, yes, it is. I unlocked it on the firmware before 4.1.2 using the method where you use the dialer and go into the network settings.
Also, for chainfire, do I have to be in developer mode and USB debugging enabled for it to work? Or can I just boot into recovery and use Odin without doing so?