5th ed is definitely the best edition to start with. Everything is fairly streamlined for new players.
My advice: you are there to help the players tell their story. You set the framework and fill in the other actors, but they will go where ever they are most interested. Planning to much can be frustrating, as they WILL think of solutions that you had not. Underplanning is just as bad. There are many blogs out there that can help you get started as a nascent DM.
Just remember that the rules are guidelines. Occasionally you should fudge the numbers to make thing more fun. Listen to your players and take lots of notes.
I'll second this as well. Everyone says "prepare" and that's certainly important, but most n00bs don't know what "prepare" means, or rather, what
to prepare. They write up a script and then get frustrated when the players don't stay on the rails. You have to learn to roll with the punches.
I recommend thinking up a few characters (at least) -- you can find reams of pre-made NPCs online -- and download a few "dungeon" maps, and then think of a few MacGuffins. If you're lost, think of a few movie characters and then just change the name & appearance. Tie them all together with motivations. For example, just a five-second idea here, say there's an abandoned castle that belonged to a sorcerer who claims to have created an elixir that could bring back the dead. That's a lot of cause for contention. Character A lost a fiance in a tragic accident. Character B just wants to sell the elixir for a lot of money (probably to A), while Character C wants to right a wrong by reviving a hero who was framed and executed, and Character D is a mustache-twirling villain who wants to bring back the Dark Lord. And bear in mind no one is sure how this elixir works, or even if it does. The whole thing could be a trap to get suckers to stumble into the castle's meatgrinder traps for some more nefarious purpose like feeding an army of flesh-eating monsters. In that case the party's struggle is to discover the truth and prevent people from getting themselves killed -- or take on the danger themselves because the motivations are too strong to be stopped.
Again, quick idea, but note that I've scripted
nothing -- especially an outcome. There isn't even anything stopping the group from joining forces with Character D to destroy the world. But just by having such a premise
exist, the group will be in the middle of a whirlwind of social activity as various motivations clash to be the first to find this elixir (and/or the truth about it). They can take sides, go it alone, try to prevent some faction making progress -- roll with it from there and let the players' actions sort it out. They may not even arrive at a consensus themselves; and you'll see the party split in their allegiance to different factions though I recommend avoiding this; party splits are tricky even for advanced DMs -- too much of bored players sitting around waiting their turn. But the fights they'll get into when trying to figure out which faction to side with, or go it alone, as the facts keep changing. . . congrats, you're role-playing.
The tricky part is introducing the characters to this mess; there are various hooks (some DMs just skip that part and go "you were hired by [someone] to do [this]" and some players in fact prefer it that way) but this way there are several -- A might not be strong enough and will need help. B wants easy money and may want to hire the group only to run off with the elixir (again, assuming it exists). The employer could even be the
not-so-long-dead sorcerer trying to find suckers to feed the monsters and thus offering outrageously high rewards for every Tom, Dick and Frodo that strolls into town while the townspeople keep warning them to stay away, stay away. In any case don't feel compelled to give the whole story all at once or even at all; they don't need to understand
everybody. Life is full of misunderstandings, and not everyone is honest. Character D might pretend to have a noble cause to deceive the group, unless s/he is confident s/he can get the elixir without help, in which case ham it up. Character A has no particular reason to be dishonest, but probably being a bit twitchy from loss, might ambush the group, only to be killed. When the players make a mistake like that. . . just play it out. Don't put them on rails; just have the world react naturally. After all, IF this elixir can bring back the dead, it can bring back someone they wrongfully killed.
The only other tip I'd provide is, have an open conversation BEFOREHAND about the tone of the campaign. In particular, is character death supposed to never happen, a real hazard, or an expectation? Tabletop campaigns can be as silly as
Monty Python and the Holy Grail or as mawkish as an anime where the life expectancy of a cute girl is half an episode. Silly campaigns are basically where the game is almost an excuse to tell jokes. Dramatic campaigns, the players are there to build a story to care about and you're there to help them, and babysit a bit if someone keeps ruining the mood. Nothing is worse than a campaign with misplaced expectations, so this is the one thing you'll want to work out beforehand and do not, DO NOT betray those expectations. You are not George R.R. Martin. If it's a Monty Python style campaign it's OK for the elixir to turn out to be a Coke, but if it's an Lord of the Rings style grind of attrition it's a slap in the face to reward the efforts and sacrifices of the group by subverting their expectations -- let them save the world or die trying.
