The first step to becoming a GM is to remember the responsibilities you're going to take on by by doing so.
Managing Expectations!
Make sure you and your players agree to things like;
How long is this adventure going to take?
What kinks are there going to be on it?
How difficult is this adventure going to be?
How rewarding will this effort be?
How much combat will there be?
How sexualized will this be?
It's okay to be vauge about some of these things, but make sure your players are prepared for what might be!
Managing time!
You'll be responsible for controlling the flow of the events. Some adventures might take longer than others, or players may drag out the situation unecessarily.
Remind players that there is a schedule to keep.
Push players along when necessary. GMs have some tools for making this happen.
Have fun!
Remember that you're playing a game with friends. The point of a game with friends isn't to win or lose, but to enjoy each other's company.
The well being of you and your players comes first, so be open to feedback and their concerns during play.
Mind the rules!
The rules exists as structure for gameplay. They're there for players and GMs to use as a means of agreeing on what to do and having fun.
Don't be afraid to ignore some of them for the sake of having fun or challenging players, just as long as you're all okay with it!