It's a little close to the wire to be posting this.
The basics, as I see it:
One more thing. If you follow the web comic Penny Arcade, I'm sure you know that Mike Krahulik, the artist, is a new Dungeon Master. There's documentation in the form of posts and podcasts from the time he was a beginning player, to his considering taking up Dungeon Mastering, through his first session, and on to what he's up to now. If your eyes are too glazed over to read that next page in the Dungeon Master's Guide, you might spend a few minutes investigating someone's parallel experiences.
- Read the Dungeon Master's Guide.
- Are the characters generated? If not, that's what you're doing this weekend, and I hope your players like it.
- If the characters are generated, have you familiarized yourself with what they're like, and what their capabilities are?
- The pessimist in me says to have another activity ready to go, so in case the actual session is starting to look like a train wreck, you can preserve some fun memories of the weekend, and maybe everyone will be willing to give it another go in the future.
- Make sure your players understand that running a first-time role-playing game session is not a cake-walk. I'm not saying you should ask for their sympathy; they just need to understand that a first session is not always smooth sailing, nor is it necessarily indicative of what the whole game's going to be like.
- I'm not familiar with the adventure, but if it's for four PCs, take some pruning shears to it. So, yes, it's a wise idea to weaken or remove the enemies.
- Make sure you understand the flow of the adventure. What is each encounter and event supposed to accomplish? How do the events and and encounters occur? How does the adventure begin?
- Rather than forcing the PCs along the pre-arranged route of the adventure, what are interesting and fun events that will happen if they deviate from the main path?
- If the players are defeated or fail to solve a challenge, do you have a plan to make their failure interesting and engaging, rather than demoralizing?
- If the players decide they're going to have their characters behave like anti-social psychopaths, do you have a strategy for talking them down?
- Most groups I'm familiar with go through one to three challenging encounters in a three-to-four hour gaming session when they play 3rd edition. Unless that's all this adventure is, or you're playing all day and all night, don't expect to finish it off in one go. Having only two PCs will speed up play, but this being your first time will probably slow it down again.
- You might want to stage a mock combat with yourself ahead of time to make sure you know what's going on.
- If no one wants to play again, it's not the end of the world. Some people don't like role-playing games, and didn't know that until they tried them. Still, your responsibility as the Dungeon Master is to make this first game as fun and positive as possible, so do your best on that account. Despite the length of this list, sometimes just shrugging my shoulders and smiling helps a lot more than fretting about whether I did enough work ahead of time.