My first experience as a Dungeon Master was, let's face it, a disaster. It was an AD&D (not to be confused with ADHD) campaign, my only experience was as a player, and my players (of which there were only two), had as little or less experience than I did. I did not know my material. I did not know how tables and stats worked. It was at about the time that I had first level characters come across a chimera and kill it in three turns that I realized something is seriously wrong here. We never did more than that one session.

Fast forward several years to my second attempt (the one I prefer to think of as my real first). This time, I'm running v3.5, and things are going smoothly. There are more players, some more experienced than I and willing to give me a hand, every so often, if I need it. I understand now the concept of hit dice and how they work. I'm more relaxed. The game is more fun. I get to enjoy the Player Paranoia induced power trip (even though I know that the darkmantle really did just get a good roll against the orc, I don't have to let on that such is the case).

What have I learned? Firstly, that it's much more fun if you have some idea what you're doing. Secondly, the players will never enter the rooms 'in order' and that they will do unexpected things, such as unleash one of your encounters on another encounter. Finally, being the DM isn't half as bad as I thought it would be.