In no particular order, here are the things that every game master needs to take into account (if not create outright). These things are also present in crpgs too; but the gm is replaced by a computer program.
The game mechanics. These are the rules and guidelines that tell you what you can and can't do as a player character; some of them pertain to character creation while others are about functioning within the physics and other scientific laws of the particular...
Game world. This is the setting of the story. It can be as small as a single dungeon or as large as a universe. If you can, be sure to include the weather and climate, native flora and fauna, the geography, the cultures and histories of sentients, any supernatural forces, and natural forces like phases of the moon or moons, tides, and ocean currents, if they're relevant. The major part of the game world are the non-player characters.
The player characters. It is best, as the game master, to guide your players through the character creation process to make sure that each character fits in with your game world and...
Plot. A plot is basically the story part of a role playing game. Some rpgs have little or no plot or storyline, while others have a lot. Generally, players prefer a lot of this. If a game doesn't have some plot to it, it is usually hack and slash.
And the game master herself. To run a successful game, the GM must either find or create the previous four elements, and corieograph all of the elements (including herself) into a functioning whole. Quite a challenge. | 
