Well, I’d suggest getting into it as a player, first - once you’ve experienced RPGs from the point of view as a PC and watched a DM work from the other side of the lines, you’ll have more of an idea as to how to Master a good game. As it is, though, here’s a little list of a few key points pertaining to what I personally thinks makes a good DM -

1. Give them a good story. In my opinion, the key to being a killer Dungeon Master is to give your players something to fight for - something to care about. If they don’t give a damn about the quest they’re on, they won’t give it their all and that leads to a half-assed campaign in which they make silly mistakes. Give them a story they’ll want to see concluded.
2. It’s their story. Don’t start up a campaign that revolves around an NPC - for instance, taking Thieves’ World as an example, don’t overuse Tempus Thales and the Stepsons or any famous character from the books. Tempus in particular can get very old, very quickly (no pun intended), and overusing him can lead to your players losing all respect for him. Tempus Thales is the God of War and Rape. They should view him with the fear he deserves.
3. Give them hope. If you make every battle they get into a fight to the potential death, it isn’t going to be all that much fun. They need to believe that they will reach the end of their quest, and if party-members are dropping like flies each time they encounter a faction of bandits or orcs, failure just becomes more and more likely. Orcs and bandits should be decent enemies, but really, your party should be able to rip through them pretty easily. Fights that really pose a problem should be reserved for the Big Bads - for the culminating climactic battles on a par with, say, the Battle for Helm’s Deep or some shit like that. Make it cinematic. Make it tense. Make it fucking awesome.
4. Don’t be afraid to alter your campaign. If your players make a choice you weren’t expecting, or behave in a way that puts them on a new path, don’t try to force them back onto your singular quest-route. Change it to suit the new dynamics - Hell, take them in an entirely new direction if you like. Never feel you have to stick rigidly to your original campaign idea - it can be as flexible and fluid as you like, and you should be able to twist and edit it to adapt to the PC’s choices. Remember that although you’re the Dungeon Master, it is their story - you might be guiding them, but it’s their choices and their IC-behaviour that really makes the campaign.
5. Be lenient, but don’t be lax. I don’t have too much time for horrifically tough DMs - if you’re punishing your players too harshly for every single little mistakes, it’s quickly going to stop being fun for them. I’m sure some players like that approach, but I’m just not in that field. Sure, punishment has to be dealt out sometimes and some skills have to take point-deductions, but don’t leap down their throats for every little mistake. Make it tough, but give them a chance.
6. ATMOSPHERE. Taking again Thieves’ World as an example, they are going to be spending nearly all of their time in the city of Sanctuary, and like their characters, they need to come to view it as a place they really know. Paint a mental picture and really let them step inside. Above all else, the Dungeon Master is a storyteller, and you need to be able to really engross them in the world they’re in. Don’t be afraid to take a little time describing their environment - not with every turn, but when the situation calls for it. Give them a world, and they will live it.

I think that’s pretty much it. Wow, sorry, this post turned out to be longer than I expected. This is just my personal way of doing things, of course - you’re your own person, you’ll be DMing in your own way and some of these points might not apply to you, but I can only give you my own views.

I hoped this helped a little, anyway - good luck become a Dungeon Master! ♥