I might as well start with the very, very basic that is somehow forgotten very easily, and becomes the root of so many problems if so. It can be contained in a few words, allow me to emphasize them:
"You have to listen to your players."
are your replying while rolling your eyes. Yes, it sounds like such a stupid idea you don't even need to say it out loud, right? As if. Let me develop for a few minutes.
At its core, tabletop RPG is a social game. By that I mean you will interact with human beings, hidden behind the flimsy screen of a character and a lot of dices (and some unhealthy snacks, in most game I've been into). A social games relies on the communication between human beings, and that alone brings so much problems. I have seen many situation that went wrong, like, screaming at each other wrong. Fortunately, we are not children fighting over a chocolate bar, we are responsible adults who can make up those situations (at least... most of us). But all theses situations came from the same origin: bad communication.
And I think this is the very first thing you should know, as a GM: your players opinions are important, and you should try to know what they want exactly. How? Well, the first thing would be to ask them what they want in a game. Coming ready with a full grindfest scenario to a group of players more interested in games of influence and treachery won't do any good.
But if you ask something like "What do you want to have in our future game?"
, there's a good chance you'll get a lot of "I dunno, I guess I just want to have fun"
responses. And that's no good. I mean, don't get me wrong, it's perfectly fine to want to have fun, but what kind of fun? Are we talking a simple combat succession with little roleplay in-between? Are we talking a railroaded plot, where you don't need to think much of the implications just to follow the GM's story? Are we talking free-roaming, exploration and random encounters? Are we talking a full drama, with lots of character interactions and very little place for dice-throwing?
As you've just seen, there's a full range of way to have fun, and everyone will have his own idea. You won't get a full response just by asking, because most players don't even know what they like. Their actions speaks much louder than simple words. So, there's no miracle here: it will take time, a lot of trials and error, before you can say you know your players. But it's worth it in the end.
Let me give you a few examples, coming straight from my gaming group. I've played with the same core of players for something like five years, and spend a lot of time at their sides just by being a fellow player rather than the GM. I can say I know them pretty well.
- My first player likes to play very original and sometimes weird character. He has no problems in building a completely outlandish character to get his thrills. To be exact, he is a big fan of old French comedy, and cape-and-swords stories like The Three Musketeers, and it shapes his characters to be like in one of those stories. He likes to create a shock by deliberately playing his gimmicks against the setting. In a world where magic exists and must be kept secret, he'll play a prestidigitator; in a very stuck-up Legend of the Five Rings Japan, he'll be rude and won't care about honor; in a fantasy world, he will refuse to acknowledge the existence of magic even though it works just before his very eyes. The problem is, he will probably neglect the rules and balance overall, creating characters that are sometimes virtually unplayable. And in a big battle like there is always one in a game, that will come to bite him in the ass, and he will probably end spamming his base attack without doing anything interesting in the battle.
- My second player also likes to play against the tropes, and in fact, subverts them with glee. However, rather than playing against the setting, he'll play against cliché: a intelligent ork hacker, a big monstrous but pacifist thing, a werewolf that doesn't actually uses raw strength to win,... He will accept a few disadvantage to make his character interesting, but on the other hand, will know the game system in and out in order to actually be playable and efficient (even though it takes him five times the work). He will also play the moral compass of the group most of the time, and even in the most violent setting, will likely try to not kill anyone if he can, going up to healing some people that don't belong in the group but weren't opponents anyway. Also, very interested in the everyday life of characters and all those little things that won't make it most of the time during a game.
- My third player likes to play with power. Not that he's a Munchkin, mind you, but he loves to have his characters powerful. That includes a whole lot of traits: disrespecting the NPC's, rubbing his ingenious combos over and over until a parade is found, being in an organization that gives him a status, seducing anything that has a pair of boobs. He will know all the rules, but only so he can screw them more efficiently. Once he gets an idea, he will not let it go, and will probably recycle it in other games as well. However, his knowledge of tactics and general culture will also helps a lot during tough games, and he will never let another PC behind.
Little trivia here: I actually described myself in one of those three players, could you find which one? ;)
As you can see, those three players have their own way of having fun in a tabletop RPG. There's neither best nor worst way to have fun, that's just the way they are, and you can't shun one of them for not being like the rest of the group. But if you are capable of dressing such a portrait of your players, then it means you know what they love and what they don't. Which means that you can give them something they like
Of course, don't be cocky: you probably won't be able to satisfy everyone in a single game. There's always that awkward moment where one of your players have nothing to do, and just start to pick up a book and skim through it. But it'll help you a lot on what you can and cannot give them in a session. If nobody is into roleplaying a love relationship other that a quick job on the kitchen table, then don't bother. If a majority loathes combat, don't spend your time sending them ninjas. That's is a simple thing, but trust me, very little people will tell you "I don't like this or that"
up front. You could even tailor some scenes for one or another player: he might not be in his right shoes during the whole session, but he still had his moment of glory just for him. And if you didn't knew his tastes, you could never have done something like that.
I know, what I ask is a luxury. It's very time-consuming, and can only be build with time and a lot of roleplaying sessions. You won't be able to do it with a brand new player, let alone a brand new group. I can say that it helps that you have played for some times as a group before you try to GM them. After all, if you were a player among them, and dedicated a little time analyzing what they did, that can only help you for the future.
It also means you will
commit mistakes in the first few games. That is perfectly normal, and the few first sessions tends to be a little rough and sketchy as a result. Don't hesitate to ask your players what they like or not during your games. Don't do it right after the game, as they will still be on their little adrenalin cloud, and all you get at that moment is praises and congratulation for that great game. No, come to them the next day, so that it's still fresh in their head but they had time to recall what they did or did not like. Criticism is always hard to take, but the few fragments you can get in those moment are part of your building as a GM.