It's been a long time since I was a newbie to BJs, so I can only give you a thought on the sore jaw thing. The jaw is run by muscles, so it stands to reason that the more you practice and stretch them the better it will get.
For the gag reflex; if you don't already, close your lips! If I hang my mouth open like a porn star, I'll gag every time, but if I work on getting the lips closed it works much better. This too will take a lot of practice if he's very well endowed, but doing it that way seems to let his prick press and glide more along the tongue instead of aiming straight for the gag trigger spot. Go slow, concentrate on relaxing, and experiment with adjusting your head to different angles so he goes down the throat easier.
You might want to consider practicing on a dildo when he's not around too, because that will take the pressure of wanting to hurry up and please him off your mind while you're figuring out what works best for you.
Hope that helps