Welcome to the hobby!
If you have the zine edition of the game, turn to the section called "GM materials," starting on page 24. That should give you a pretty good overview of how running the game works. In brief, as the GM you'll be creating a story at the table, using the framework and inspiration provided by GSTR—and, of course, drawing on your fellow players contributions too!
If you want to see some examples, I recommend checking out this Youtube playlist of recorded sessions of the game: https://www.youtube.com/playlist?list=PLp2rr7I2jFTlcOHYF9PNj55wQ7owv4rJL