When you start, think of it like writing a novel. Once you have the basics down, then you get into the nuts and bolts of the game and start drawing maps of the goblins caves and so on. And try to figure out what your players want. Do they want a lot of battles, or more talking? Do they want to solve a mystery, stop an assassination, or get into a political intrigue? Do they want to quest for an object or to deal with an evildoer?

Sorry, I have to run, I’ll try to come back and finish my answer.

If it’s your first time DMing (dungeon master) or running a campaign then its best to get a pre-made campaign designed by the makers of the rpg you are playing (D&D has lots of fun ones)
To get set up you need:
Pencils
Paper - lined, blank and graph if you can
Dice
Something to write on

To stay organized get a binder with dividers, you will want a copy of each players character sheet (optional) and a place for maps, puzzels, campain details, each having thier own section for easy reference, keep all your gaming supplies in one area/bag so they don’t wonder off. Sticky notes are also good for quick notes you can pass to players or mark on maps and screens.

Get everyone together to create thier characters this way you can bounce ideas off each other and get a good feel about how you want your characters to be and interact with each other. Make the first day just character creation so if there are any questions the DM is right there to answer them. Arrange a time that works for the whole group and encourage everyone to bring a drink and munchies for gaming. Talk with your group about the party make-up and what is and is not aloud (races, alignment, deities)

It’s always a good idea to have a few character ideas written out to bring with for character rolling, and in some cases a back up character incase of death. If everyone is on the same page and the Dm has expressed his wishes and everyone is happy with thier characters then you are ready to game. Character creation can take up to 2 hours depending on the detail and knowledge of the players.

Gm’s have alot of work ahead of them. Not only must they devise a storyline and rules, but they need to make charachter sheets for enemies, npcs and you should probably prepare some extra enemies and scenarios just in case. This can be hard to know where to start and what to do. The best thing is to first find out your campaign setting. If you model this after something familliar, you can more easily create what you want to happen. For instance, osme of my favorite campaigns have been modeled after some popular entertainment. I once had a redwall campaign where I was a ninja otter who could fight in handcuffs. We once did an Xmen/SHIELD campaign where I was a mysterious Gman. We have modeled campaigns after starcraft and warhammer, but anything is possible. Star trek, Firefly, Lord of the rings or even Conan the Cimmerian can all serve as possible universes for your game. After you figure out your setting, you need a story or something for your charachters to do. Puzzles are good. Noone wants just wave after wave of enemy, and a mystery could be a fun thing, that will prompt your players to think logically and deductively. Once I solved a puzzle (and got a free point) when I decided to have my charachter learn an enemy language. Once you have your storyline, setting and rules then the real work begins. Making charachter sheets, balancing rules, writing tables and thinking about how to make the game more enjoyable. That is why it is preferable to have alot of experience in tabletop gaming before you attempt this, because a thourough understanding of the rules is necessary to design your game and charachters so it is not too hard or easy. (favorite quote from freaks and geeks after bill complains about dying “ohh, I assume you would rather have had to do battle with kittens and grandmas”). You must know all the inns and outs to make a good gaming experience. This is why I am a crappy DM. I can come up with good stories, but I havn’t played enough to have a firm grasp of game mechanics, and the gameplay suffers. But if you have a good fantasy/sci-fi story in mind, and you know the system you plan to use (GURPS, Alternity, DnD etc) fairly well, then it should be a breeze. Get lots of graph paper for maps charsheets and tables.

if this is your first time running a game start small
you can expand your world as you gain experience to better challenge your group
set up a time to get everyone together to build their characters
and ask any questions about what you’ll permit for a character type or class
if you’ve been playing with your group for some time you might have figured them out to some degree as to what type of experience they enjoy most
some love to bash heads
others drool on a good puzzle or mystery
a few might like the political intrigue
you can’t please everyone at once but balancing things out will keep everyone happy
example
start with some intrigue that the party stumbles into
while they are sorting it out a bunch of baddies bust in with weapons swinging afterwards while the party is healing up they can look over the clue they found to lead them onwards

round up all your gaming materials
your going to need plenty of note paper
post-its are handy both for quick notes and for info to players
a 3 ring binder with lots of trapper keepers in it will keep everything in its place for easy access
spare character sheets for new players
info on the current party
NPC’s/monsters
locations of note in your world
maps of particular buildings and towns where the party will frequent
especially if a fight breaks out
a vinyl grid mat for playing out encounters or premade building/dungeon
pieces
with dry erase markers and cleaner
dice and plenty of them for new players and for those who occasionally forget theirs
a DM//GM’s screen for the game you will run as it has tons of very useful charts on it for easy reference
if your game doesn’t have a premade one you can scan all your needed charts out of the books and paste them together on your PC then print them out and glue them to some heavy cardstock in a trifold pattern
using D&D for example the following charts
AC
THAC0 .
saving throws for PC/NPC per level and for items
spells per level listings
proficiency/feats per level
treasure listing
poison types/saves
critical damage table

always be prepared for the unexpected
the party will easily trash your best laid plans by doing something off the wall or even going in a totally different direction
this is roleplay at it’s finest