Tips for first-time DMs (from an old hand):
- Run your game on a simple system the first time through. Something like West End Games's Paranoia or something out of White Wolf. This keeps you focused on the game and not the rule set, and helps to keep you from becoming a rules lawyer.
- Don't try to be mean. Killing the players quickly and painfully is not the goal. Many first-timers will cackle evilly as they send an armada of hover-tanks against a lone unarmed character. This will frustrate your players, and decrease the fun factor.
- Reward your players. Another thing your PCs will find frustrating is completing a great task, and not getting much for it.
- Don't over-reward your players. Newbie GMs will sometimes give their players stuff that's just wayyyy too powerful. This means you have to come up with something better for next time. You want to slowly give out stuff, not throw them a +17 Sword of Chopping Enemies Into Tiny Bits.
- Use humor. A little levity can really take a tense edge off of a long fight. Even if the joke is bad, you can get some serious guffaws.
- Reward your players for the right reasons. Don't reward the players for killing something; reward them for killing the enemy in character or in a creative manner.
- Plot. The plot is all-important. If you want to spend your life rolling dice, that's fine, but there's plenty of great wargames already out there.
- Having a non-player help you out can be useful. One of the key traits of a good GM/DM is that they can think on their feet. Co-running it will let you still have an enjoyable game while learning the ropes.
- Over-prepare for your first game. I don't mean you should be rigid and inflexible (i.e., like Al Gore's posture or George W. Bush's mind), but think of as many things that your players might try as you can, and be ready for them. As you learn to adapt quickly, this will become less necessary.
- Your first time will be pretty bad. That's a fact, you'll have to live with it. Therefore, don't worry about it at all. Loosen up.
- Don't under-do or over-do descriptions. If you spend a half-hour describing a room, your players will get bored. If you describe someone as merely a "tall man", they seem like a cardboard prop.
- Prepare in the physical sense, as well. Plan for at least one break in which everyone can go outside, sneak a smoke, or maybe even make a Quest To Go Buy Skittles. Have food/drink. Pizza and soda are preferred by some, other like chips and beer. Regardless, something like that is a necessity. Have pens, paper, dice, extra character sheets, books, chairs, a large table, etc. Have something unrelated for the players to do. I've got a pool table, and it makes a good diversion when I'm in a GM-PC conference, or while someone is still making a character.
- Discuss your player's characters with them before starting. Maybe this would only take 5 minutes, but you gotta know what you're dealing with. Force the players to stick to the characters.
- Have fun. Have fun. Have fun. There is no rule more important than this. Have fun. Did I mention that fun was the key factor here?
Anyway, I hope that's of benefit to someone.