Being a game master can mean many different things and entail a lot of different work. You could be more a moderator, allowing your players to tell their own stories and create their own conflicts. You could also be a more of an overlord, creating unique challenges for your players to overcome. The most important thing to remember is that you have the power, the choice, to do whatever you really want and can do.
Choose your own adventure
The first major thing you will want to decide will be what role-playing system you want to use. The first major question you will want to answer will be the genre. Do you want to play in a fantasy setting? With swords and magic and dragons? Then you may want to consider Dungeons and Dragons fifth edition. This system is actually pretty easy to grasp compared to others out there and still has a ton of fun things to do. If you are looking more to science fiction setting, then one system that I would have to recommend would be Fantasy Flights Star Wars roleplaying system. Now, this is actually a series of three books each outlining a different part of the Star Wars galaxy. If you want a sci-fi setting that is not Star Wars, I would suggest Eclipse Phase, a pretty easy to pick up a system with a lot of interesting ideas. You do not have to always have a system with a pre-made genre, Fate Core is an amazing system that has no setting, and so allows you to make whatever genre you want. In the end, just find what interests you and then you cannot go wrong.
What does that mean?
Before I help you dive down the rabbit hole lets define what a roleplaying system is. This is, for all intents and purposes, the collection of rules that you and your friends are playing in. Not only that but some systems also come with pre-built worlds. By this, I mean that some systems have things like a pantheon, cities, countries, planets, factions, and/or technologies made for you to use. Keep in mind that you never have to use any of those things at all, as you are the Game Master. (I won’t tell anyone)
Preparing for your journey
No matter what you have to remember that you also cannot play a roleplaying game without players. Find the group of friends that want to play, or make some new ones. Online resources such as Roll20 allow you to even play online or at least network to find people who share your passion. The best part about roleplaying games is that you get to make them your own, there is no program that dictates what the next level looks like, or that limits the dialogue, every choice you make as a Game Master is your own, so just embrace that and you can’t really go wrong.