There are a few things I think are important to keep in mind for the new gamemaster running a game for the first time (or anyone, really…)
1) Preparation: I know a lot of GMs are down on preparation, but I think it’s important to have a decent working understanding of the rules, and to have an idea for a basic plot or adventure for the group.
Knowing the rules at least well enough to know the basic mechanics, say, attribute+skill+die roll vs. a target number is essential. Yu don’t have to know the specific modifications aiming, or called shots lend or take away. you can fudge those the first time if you can’t find the rule right away.
Having a basic plot for the initial story makes it more interesting to the players and is more likely to lead to enjoyment. Players like it when their characters achieve something. They might not follow your mental script, but have a few essential scenes — waypoints to the final event — will help push the plot without feeling like you are railroading them. (More on that in a moment.)
2) Confidence: Okay, you’re new to the rules yourself, the adventure feels a bit kludged, and you’ve got, essentially, stage fright. Don’t show it. Loosen up and enjoy yourself; this isn’t a job. As military officers are taught — look like you know what you are doing…especially when you don’t.
As long as things are happening or characters are interacting well, they’ll often miss glaring plot holes they might notice in a movie or book…because they are (kinda) living it.
3) Be flexible. The players aren’t always going to do what you want. If you need them to pick up on Clue A at a scene, but they don’t bite, roll with whatever they are doing and put the clue in the new location or scene. Maybe they kill the bad guy early or they go through your cool action scene with mooks (and this will happen with annoying frequency)…now they find out bad guy is actually the lead henchman for the new bad guy. Okay, they got through the action scene that was supposed to be tough like they were walking down the street to catch a bus; let them bask in their reward and at the next scene hit them harder, or perhaps make the reward less impressive. “We took on the entire gang of Shen Ho to find…a shipment of bananas? Where’s the [insert prize] that was supposed to be here?” Nearly dead mook, “Shen came and got it an hour ago…they’re loading it on a truck for…ack!”
4) Have fun. If you aren’t having fun GMing, hand off the responsibility to someone else (if someone else will do it.) It’s a game.