My tip to you is to read, read, read the material and become very familiar with it. Not only the rules but the adventure you're running too. Especially the adventure. Any RPG runs a lot smoother when the GM knows his/her stuff and isn't consulting a module or their notes every thirty seconds. You don't need to memorize the adventure verbatim, but you should know the major details of the plot, key encounters, important NPC's, etc beforehand.
I would also suggest taking a little time to consider what might happen if your players decide to go off the beaten path in a direction not covered in the adventure's text. Obviously you're never going to be able to think of every contingency (a group of players can be wildly chaotic, even on their best behavior), but you'll still be better prepared to gently guide them back towards their objective if they're running astray.
On the other hand, don't stress too much if things aren't perfect. It seems you're all going to be D&D rookies going into the game -- there might be an occasion where this rule or that rule is called into question or disputed. Instead of delaying the game for fifteen minutes to flip through the rules, make a gut judgement there and then as the DM and stick with it for the rest of the game for consistency's sake. Then later you can go back and look up the 'proper' rule to use next time (or not -- house rules are very common in practically every tabletop RPG you'll play).
Best of luck. It's a lot of homework involved as the GM / DM / Keeper / ref / whatever, but it can also be rewarding and a lot of fun too!
