The first time I DM'ed was a Planescape adventure. I'd been playing for a year, and it was ten years ago. I DM'ed the adventure out of the Planescape boxed set, pretty much as written save for the planescape slang, "cant". I had a gnome fighter/wizard with Str 18/00 and Int 18, which I still remember because he rolled in front of me and the became the dart-throwing killer machine of doom for the two fights they had.
I made a big mistake because when the party annoyed/upset the helpful plot device lich (who had captured them), I disintegrated one of the players. Thus endeth the session and my first campaign.
I still loved it, just as I do nowadays (but I think I've become a little bit better at this).
I took three important things with me that day:
-> Be flexible enough to allow for a lich having prepared Hold Person even if it's not on the character sheet
-> If the party is sitting around, doing nothing despite having incentive to do so, let something unexpected happen (here, I had mephits appear and attack out of thin air)
-> If your "normal" DM has a control fetish, he'll walk out on your campaign first chance, a lesson that got hammered down in campaigns #2 and #3.