The player who acts as The Keeper of Arcane Lore (or simply, the keeper) becomes the game moderator. Perhaps using a published scenario, or creating one of his or her own, the keeper knows the entire plot of the story and presents it during play, incidentally taking the parts of all of the monsters, spooks, and sinister or ordinary people that the investigators meet. The keeper has the responsibility for preparing a scenario without bias. He or she must make the opposition smart and mean, or there will be little challenge for the players, and they will be bored.
The keeper needs to understand the game in order to be able to answer player questions and to be able to present the material fairly. To be a keeper, read the rules on creating investigators, the game system, and sanity, and then go on to read about magic and the Mythos. You'll find individual monster and spell descriptions in the reference section of our Call of Cthulhu rulebook. Those entries can be consulted, and do not need to be read through and memorized. (That would be quite a feat!) Skim the keeper's lore chapter.
Then choose one of the short scenarios from Call of Cthulhu rulebook. Read it, then invite some friends over. Have them roll up investigators together (be sure to photocopy enough investigator sheets first). Or photocopy the ready-made investigators at the back of this book, and let them each choose one—all they need are 60 more points of skills to help presonalize their character.
Summarize the rules for the players, and supply a photocopy of the game tables at the back of this book. When you’re set to play, don't worry about making mistakes the first time around. It is your privilege to make mistakes. While you are learning how to invent your own scenarios, use the ones in the Call of Cthulhu rulebook or consult one of the many scenario supplement books available. Often printed scenario thoroughness and extra information can be welcome.
As you need, consult the reference and resources sections, or read pertinent sections in the keeper's lore chapter.
With the purchase of the Call of Cthulhu rulebook and some dice, you have everything needed to play the game.