[MENTION=6675439]Xx8_bit_ninjaxX[/MENTION]:
1. The DM is always right, even when he is wrong
2. The DM is always right, especially when he is wrong
3. Win if you can, lose if you must, but always cheat
4. Use Rules #1 through #3 to make sure everyone is having fun.
Without Rule #4, the previous rules just kinda make someone a jerk. So use them to make sure everyone is having fun! Oh, and by cheat I mean making up skill checks on the fly, adjusting the pace of battle (add more grunts or have them run away is handy, so is adjusting HP if a battle is too long or not long enough), and be willing to have situational house-ruling (Etc: Let's go with {this rule} for now, and we'll look it up later) to avoid rule arguments during the game. An occasional +5 vorpal clue bat may need to be applied. This will take some practice to get the hang of it.
Also, the Gamemaster's guide has a lot of good advice, charts, and tools.
Now, one thing to remember as a new DM is that you are going to make mistakes. That's okay. The big thing is to roll with them, and to have patience with your players. Remember, your players are clever little munkeys and are out to thwart you. Let them. Don't try to force your players to solve a problem with only one or two solutions. They will come up with their own, and blindside you.
Another thing is preperation: It is better to be prepared to improvise than to try to map everything out. Not only does it take less time, but it leads to a more dynamic feel to the world rather than it being railroady.
When it comes to plot, I find it best to think up a few general points, and let the details be filled in during play. Sooner or later the players will send your plot off the rails by doing something unexpected (the clever little munkeys that they are), so it is best to be flexible. Try not to take it as a personal affront. The more scripted the story is, the less flexible it is, and the less fun it is for players. Players like to contribute to the story. If you're just wanting them to act out your play, write some literature instead of DM.
Everything else varies by play style, and these are just a few of the pointers. You'll find your groove if you stick with it.
[MENTION=6672910]Dingo333[/MENTION]: Weapons of Legacy worked sorta like that.