Have a good grasp of the rules, but don't be afraid to ad-hoc your own rules if necessary.
If the players think of something that you didn't think of, or come up with a way to stack the game in their favor, go with it.
I had to run a game for a party of six before, four of which were newbies. I ran each of them through a minor battle -- two or three of them versus two bad guys. Just so they could get the hang of rolling dice and adding the appropriate numbers. After that, I threw them in full-swing and they kept up. I kind of rushed through 1st and 2nd level, though, because I don't you start to get the feel for different character clsses until the casters have a few spells to burn and the fighters have some hp to burn.
Oh, and if any of the players have some experience, use them to lighten the load teaching-wise. One knowledgeable player can really help out the rest a lot.
EDIT: And feel free to always ask more questions! We enjoy answering them.