But this is a subject I've been asked about and want to get into. This will be a long one, so I'm splitting it over at least a couple days, this post will be related to the preamble for getting a game going, Figuring out what you want and getting the materials to do it. Next up I plan on getting it set up to begin a game, and then possibly keeping it going once you have started in the last part.
The disclaimer out of the way, let's begin a magical journey.
The first and most important part of getting into running a game is to find a system you want to use. Typically this is best solved by what you and your group want to run. I don't think there's a single genre out there that's not been games these days, some may be obscure, but they're probably out there and enough looking will find either exactly what you want or something you can bend into the niche you're after.
- For this, It's very important that you as a GM want to run this system. If you and your friends and you agree on hack and slashy fantasy, but they all want Dungeons and Dragons and you hate that system, you'll go mad. No one deals with every single inch of a system more then the GM and the top priority is systems that you'll feel will work best for you. Both GM and players may need to negotiate a bit, but you should stand firm on this.
- A personal favorite aspect of mine about RPGs is that there are no true starting systems. There are systems that are so simple that one dice roll can determine a whole fight or use a jenga tower in place of stats, and some that require you to do math and determine the fuel needed to power the engine of your mecha, but nothing is really RPG 'training wheels'. Your sole requirement: understand the game. A complete memorization of the core book isn't needed, but knowing what page to go to for specific kinds of obscure rules, and strong recall of the common ones. So long as you know the details on how to run the game, and your players can learn it, you can run any kind of game. You don't need to learn on something you don't want.
First though is getting the books you actually need though. Almost every game ever uses the same format for this, fortunantly, and they're best summed up the way Dungeons and Dragons does it. The big three: Players Handbook, Monster Manual, and Dungeon Master's guide. These may be combined into one or two books per a game, but this is the most common system.
- The players Handbook, also commonly called the Core rulebook, or simply named after the game you'll be running. (E.g. Mutants & Masterminds, Cthulhutech, Call of Cthulhu.) This is of top priority, it will contain the rules for making characters, killing things, resolving actions and all that good stuff. If anything this is the one you'll want everyone to have a copy of. Despite its name its for GMs too. It's the accumulation for the basic rules. The core book is often the most combined with the other books, and is in essence going to be the most important and referred to book in the series.
- The Monster Manual: This is the accumulation of monsters, basically. A whole book of nasty creatures to send against players. In a lot of genres this may be less applicable a thing, but once you start running a game you'll see how very handy it is to have the stats of enemies pre-assembled and ready for you. These tend to be tested well and by studying them, you can best gauge what will be a challenge and what will wipe the floor with your players. This is often combined with other things, like the core rulebook. Do note, if you plan to use something in these a lot, warn your players. Its considered very bad form for them to memorize these types of things, especially if it gives them insight they shouldn't have such as weaknesses.
- The Dungeon Master's Guide: This is probably the most abstract of the books. It tends to give two things: Advice for dealing with game specific issues. Powers which can cause problems in superhero games, they also often contain information on the world, details for building parts of a session and the like, and variations, Optional rules you can replace things to make the game suit you better. Some of these are very good, some are less so.
- Other books: Again using the D&D as an example, here's one of its 'weaknesses', D&D has a LOT of books and people are often overwhelmed by how much stuff they'd need for running a game of it or another popular RPG. This has elements that are true, obscure book specific rules are a annoyingly common thing and may help you or a player immensely. The general reality though, is that they are only needed for specific niches. Using D&D as a model, if you want to run a game using D&D's mechanics set in the desert, the D&D book Sandstorm is recommended, it has specific information about that environment, monsters, creatures, lore on D&D's official settings deserts. There's similar books for specific monster kinds, undead, dragons, alternate planes of existence, alignments like evil and more. When you have other choices for books you can get, you don't need them all, but if it's very relevant to your game, it can be worth looking into.