At times, the gamemaster has the most fun in the game, but this comes with the largest share of responsibility. Each player must share the spotlight with all the other players, the gamemaster is constantly interacting with all the players. Using a published scenario or one they have created, the gamemaster narrates the game universe and acts as the player characters’ opposition. That opposition must be challenging and entertaining (or the players will be bored) and it must be presented fairly (or they will be outraged); otherwise the game will not be fun (which is the primary point of roleplaying).
The gamemaster should read and be familiar with these rules. Know the general procedures for combat and powers, but it’s not necessary to memorize everything—most questions can be answered as they arise.
As for scenarios, there are a vast range of scenarios for many different settings and games. Chaosium Inc. has published many adventures for Call of Cthulhu, RuneQuest, and other game lines, most of which are based off the Basic Roleplaying system. Other games are plentiful—and converting a scenario from one of them to Basic Roleplaying is fairly easy. Ideas for scenarios are also easy to come by—almost any film or book with some aspect of danger and excitement can be turned into a roleplaying scenario.
Ultimately, all that’s required is to come up with a story, write up some foes or encounters, and invite some friends over and have them create characters. Summarize the rules for them, and it’s time to play.