I'll second the GM's guide for Rolemaster is an excellent book for new GM's.
1. Remember that, while you want to be having fun, your primary goal is to make sure your players are having fun (because without them you won't be no matter what). Find out what your players enjoy...
- Combat? Role-play?
- Accumulation of wealth or fame (or both)?
- Do they think adventurers are above average examples the population (stats/skills) or what they think they're like anyone else and it's what they do with what they have?
2. Your players will often derail your best laid plans. Learn to anticipate this possibility and know there will be times you have to wing it. However, when possible, keep in mind you can sometimes potentially 'move' your planned event into the players path.
3. Don't leave players with nothing to do for long periods of time. If someone is captured or incapacitated figure out a way to not leave them twiddling their thumbs while the rest of the party plays on. Resolve what has happened quickly, let them play the NPC, or maybe even pause the session and continue on once it's been resolved.
4. Rules aren't written in stone. If something isn't fun, change it. Our group will occasionally stop a session and hammer out a new rule (among everyone) that addresses something no one is happy with.
5. Somewhat related to #4: Remind players if they think they should be able to do something they shouldn't or you don't want them to they're quite likely to be on the other end of that equation eventually and will they be regretting it when that happens?