Basics of Game Mastering
First and foremost, the Game Master (GM) is a player at the table, alongside the Players (PCs; Player Characters) themselves. This is important to keep in mind as the role of GM can often come to feel like more of a chore, or a job, if everyone at the table does not ensure that the GM is having fun too.
With that out of the way, the role of the GM is quite simple, they facilitate the world within which the PCs are playing. As the GM, you are the Non-Player Characters (NPCs) which the PCs interact with, the enemies they fight, be they sapient or ruthless monsters. You are for gods of the world and the forces of nature themselves.
Calling for Rolls & Determining Outcomes
Once you have communicated to the party what their circumstances are, the most common thing you will do as GM will be to call for palyers to roll dice, then determine the outcome of their actions. During combat, when roll sneed to be made and the rults of player actions are largely define in the player's Compendium, however outside of combat, you will need to determine when it is necessary to involve dice, or when is can simply be determined whether or not an attempted action will succeed.