As a teaser from my FANTASY HACK m20 game, here's a portion of the text in Appendix I; basically it's meant to indicate a quick summary of my GMing advice of many years, distilled down to just a couple of pages worth of text. This should be pretty basic stuff to anyone who's run for any length of time, but the audience that this is written for is primary newer GMs.
Successfully running a game of FANTASY HACK m20 (or any other role-playing game) is a challenging yet rewarding endeavor. Besides; someone has to do it, otherwise there's no game for anyone! If you accept the challenge of being the Gamemaster, there are a few things you should know. This section has a very small bit of advice, based on my own experience, followed by a fair number of tools that can help you. In no particular order:
You're a player too. Although your task, and therefore what makes the experience rewarding and fun for you is perhaps a bit different than for the other players, this is still a game and you should be enjoying yourself too. GMing is not a chore; it's not a job, it's not what the guy who gets the short stick has to do. If you are not motivated and having a good time, the game will suffer because of it for everyone. If so, consider giving the reins to someone else.
Be fair and be consistent. One of the things that the players need most is feeling like they can make decisions for their characters based on reasonable risk assessment. In other words, they need to feel like they understand the way the world works (and most likely they expect it work like the real world does.) Although this is one of those "perception is reality" kind of things, especially on a highly rules-light game like m20, the players will rely solely on your judgement about how likely things are to be successful. If they can't get a handle on that because your rulings and DCs are inconsistent, or if they are consistent but out of whack with their expectations, either one, it will create the strong impression that the game is arbitrary and therefore unfair, which will dramatically reduce the enjoyment that your players feel.
Be varied and interesting. There is a wealth of sources in terms of ideas for your adventures. Don't ever feel bad about borrowing from any and every source you can imagine; books, TV shows, movies, video games, whatever. Just don’t borrow the same kinds of elements from the same kinds of sources. Even Gary Gygax wrote (although this is often forgotten) that the game was not supposed to have been mere dungeon-crawling, and some versions of the game stressed doing other things (not that this was often appropriated by the players.) FANTASY HACK m20 is flexible enough to be used for all kinds of activities, and it actually is not designed specifically to be a dungeon-crawling game at all. In addition, if you pay attention to your players, you will before long find it easy to judge when they are engaged and entertained, and when they are more bored or frustrated. Pay attention to this and give them more of what they like and less of what they don't. They may not all be on the same page about what their favorite aspect of the game is (and they may be in different moods to do different things at different times anyway) but some situational awareness is crucial for good GMing.
Be generous and say yes. Although I personally dislike games that are overly concerned with the acquisition of character wealth and powers, in general, players tend to be happier when they get what they came to the table for, rather than feeling like it's denied them. This doesn't mean give them "stuff" necessarily; but it does mean allowing them to indulge what they want to do as a character. FANTASY HACK m20 is meant to emulate swashbuckling action stories. Think of a well-known example like the Star Wars franchise. Do the characters ever get bogged down looking for equipment that they don't have access to? While getting passage to Alderaan is a key plot element of the first movie, it's easily accomplished. When Luke needs a lightsaber, he has one. When the characters have the opportunity to have a speeder bike chase, they're readily available. How does Luke even get his X-wing that he flies for most of the movies? I dunno. It's there when he needs it. This is the kind of story that I intend to emulate. Hoarding of gear, doing tedious accounting and shopping are not at the heart of this kind of story; they are things that are typically breezed over because they are tedious and boring. Now; some players actually do enjoy that kind of thing, so I don't recommend excising it entirely. But I do recommend a focus more on the action, role-playing and the solving of interesting problems than I do on making things arbitrarily difficult for the characters. That's the spirit of swashbuckling adventure stories, after all.
Let the PCs dictate the game. Don't overplan, because you will tend to get locked into your plans the more time you spend on them. This isn't your novel that the other players get to have a minor role in. This is their game, and you're supposed to represent the environment and the setting. Let them be the stars of the game, not anything that you’ve created. Let them decide what kinds of characters and what kind of party to create; don't passive-aggressively punish them for not picking your ideal of a "balanced party" or whatever.
Don't give them simply one solution to problems and ensure by fiat that anything else fails. If you are too prone to trying to not let the PCs have their head, as the saying goes, then maybe you should rethink being the GM. If that’s the only way you can enjoy the game, then you are probably not equipped to be the GM. Being a successful GM means always remembering that it’s their game. You'll have plenty of interesting and fun things to do, and honestly, you'll probably be a great deal more entertained by seeing what they come up with then you will be trying to ram them into your own ideas of what they should do.
That said, it's also my experience that few groups have enough initiative, especially at early stages of the game, to know what to do from scratch if you give them total freedom. Usually they will wander around aimlessly and even with a great deal of frustration "trying to find the game." Once they are able to anchor themselves a bit more into the setting and their characters, they are much more capable and willing to take the reins, start making things happen that they initiate, and pursuing character goals that they themselves have set, rather than plot goals that you have created for them. So ease them into it, but when they're ready to take control, absolutely let them do so.
Be prepared with things to do if the players seem lost, bored, or just need some kind of motivation. To quote Raymond Chandler, "when in doubt have a man come through a door with a gun in his hand." An ambush by brigands, thieves, highwaymen, cut-throats, or dangerous wild animals is sometimes just the thing to get the game going again when it flags. Have a list of names appropriate for your setting that you can draw from to give NPCs that you didn't anticipate the feel of being more than a hastily constructed expedient. If the PCs ignore threats or certain events, in the back of your mind think about what is happening while they're not intervening. Make their decisions (or lack thereof, as the case may be) have consequences that they can see in game. Maybe they still won't care, (although you should take that as an indication that you're probably not presenting them with the kind of game that’s engaging or interesting to them if so.)
More likely, they'll take the perceived failures personally and be more motivated to keep them from happening again. Nothing gets players more motivated than a rivalry with an NPC that has gotten the best of them at least once in the past. In short, make your setting feel like a real world, not just an environment for them to interact with. This is the big benefit of table-top RPGs over computer ones; you can have flexibility to do all kinds of things that a computer programmer could not anticipate, and you can react to PC actions that they wouldn't even be able to do in a computer game. Do not make the mistake of sacrificing this advantage for your own convenience; your game will suffer from being too much like a computer game… but without the nice graphics.
But again don't over-prepare. You don't need gigantic campaign settings the size of a continent. You don't need a lot, actually. A very brief outline of what you think is likely to happen over the next session or two, including a few details about some NPCs, monsters, and locations that the PCs are likely to encounter is usually sufficient. I rarely type up more than a page of outline, and it usually ends up lasting for several evenings worth of play. But in order to do this well, you simply have to practice. Don't be afraid of not running the best game ever when you're starting. You'll probably do better than you think, and even if you don’t, you’ll get valuable practice and experience and be better at it next time, if you pay enough attention to your group to notice what went well and what did not.
The Secret Roll. As GM, you probably need a few details about your characters—a single line will suffice, but have the character and player names, their stat modifiers, AC, and skill modifiers and level noted at least. There are always times when as GM you will want to make rolls for the character that the player is not aware of, or at least cannot see the result of, because a failure would give them knowledge that their character could not have. A great example of this is where another NPC is trying to sneak up on the character, or when the characters are traveling and may get lost but not realize it while traveling through the wilderness.