Being a Dungeon Master
I'll keep this page simple and sweet as it's really up to the individual on how a Dungeon Master (DM) should play. A DM is essentially the game-keeper, the rule-maker, the overall "god" of the world you're playing in. They decide everything, from what should be allowed, to what monsters you run in to. However, it's important that, as a player, you make sure your DM knows if you are having fun or not, and how to change that if necessary.
As a DM, yes, you are pretty much the end all be all, but never let this power go to your head. DMs who play like they are some merciless god that their players should fear are no fun. You should always let your party choose where they go, what they do, and what missions they take. You'll run into situations where the party doesn't go on the story line you spent days crafting, and you have to be ready for that. Quick thinking is as important, if not more so, for the DM as it is for the players. The players will keep you on your toes, and you shouldn't get mad at them for that. Just a tip between you and me, if there really is a story line you want them to do, you can always change how they receive it. In other words, if they don't go specifically for it the way you want, you can always change it to where they find it out during whatever other thing they wanted to do. Just segway that past story into they one you really want them to go on.