A typical session can last anywhere from 2 to 4 hours once everyone is settled and ready to play. The biggest limiting factor here is the GM’s ability and willingness to continue running the game (something you and your group should discuss openly and honestly).
A GM should have a firm grasp of the basic rules, as well as a starting scenario in mind, but you don’t need to have everything memorized.
All the usual social rules apply (to your Players and especially you as the GM): be respectful and attentive, and everything else will fall into place. Spend a few minutes getting acquainted with each other, reviewing the basic rules, and you're off!
You don’t need to have every rule memorized, but as the GM you should be familiar enough with a game’s core principles to teach others how to play. Be prepared to answer questions and figure out the rest as you go!
While more experienced players will have their own dice, pencils, paper, and character sheets, first-timers may need a hand with all this. The typical Starter Set for a TTRPG has a set of pre-generated characters for your players to choose from, and jump straight into the action.
If your game session was a video game, the Players would be the main characters (like Mario or Lara Croft) and you would be the software engine simulating the world (like the Mushroom Kingdom or… Egypt?). Think of yourself as a collaborative referee or facilitator, more than an opponent. Even when you’re throwing challenges and obstacles at the Players, the GM is always available to answer questions and help the Players understand what’s happening.
Not right away, and maybe not ever if it’s not something you’re interested in. As you and your group get more comfortable together, you may find yourselves using different voices, dramatic gestures, or sound effects to describe things in your game. This will start to happen naturally, but this is not a requirement for any of you (especially at the beginning of your first session!).