You've been DMing for a while now, let's suppose.
Let's set the stage.
Imagine a grizzled, old Wizard, staring down from a hill.
There are a lot of excellent Dungeons & Dragons setting books out there that will give you all the details you need to run a game in their setting, and those settings (also known as campaign settings, world settings, campaign worlds and/or simply campaigns) are often excellent.
Sooner or later, someone always suggests this one.
Most people are familiar with the general idea of the badges you earn in Girl Scouts -- learn about a topic, tick a couple boxes worth of requirements, and you've got yourself a cute triangular patch to iron onto your vest or sash.
Everyone runs their game differently, and I don't mean to even try and tell you this is the way to be a game master/DM/whatever term you use for the person who runs the game here.