How to Introduce a Role-Playing Game:
Starting a new group of people in an RPG can be a daunting task. I personally initiated a select group of my friends into D&D 5th Edition. Our group has been dabbling in 2E & 13 Age, but I am much more familiar with that system. Some of my advice will be 5e specific and may have to be altered for different RPGs.
1) appear to know the rules. The perception of knowledge is far more important than actually knowing the correct rule set. Still be familiar with the basic rules in the book. But also be able to make quick decisions. Nothing is more alienating than pausing in the middle middle of a tense moment to check the rule book. It breaks the suspension of disbelief. Keeping the flow of the game is paramount. You have an opportunity here that is relatively unique: nobody should be a rules lawyer. You have a lot of freedom and trust from your players. The onus is on you to make the session fun.
2) use pre- gens. I know you love making characters. I know you want to share that experience. However, first time players find it stressful and scary, or fail to foresee the consequences of their decisions. In 5E specifically, building a sorcerer, wizard, or Druid is quite complicated. If you are starting at or above 3rd level characters, sorcerers have to choose Metamagic. There are about ten choices, but in actuality if the player does not choose “Twinning” spells, they will be underpowered in every combat. Character creation is laden with these high impact low information decisions. By removing those choices you can start playing the first session, and have a relatively balanced party. Again, the goal is to accelerate play and increase the level of fun.
3) have the adventure prepped ahead of time. Your inexperienced players appreciate guidance. A premade adventure, or an entirely synthesized adventure will have a clear plot, and allow a controlled progression of skill. Players are likely uncomfortable helping create the world. That will come with experience. To start though, having a full adventure prepped allows for faster play as all creatures, stat blocks, and maps are readily available. The preparation takes creative pressure off of you the DM during play, leaving you more cognitive space to focus on your player’s fun moment to moment.
4) Be adaptable. This is the most difficult of the advice to define. If you have a sneaky group, point out opportunities to be sneaky. If your group is bad at making decisions in the sandbox world you have created, give then one to three specific options for action. If the group enjoys fighting, get them from fight to fight quickly. If they prefer role play, enrich the world with more and deeper characters. The caveat here is to maintain the prepped adventure, but modify how it is presented based on the groups reaction. I currently have started two groups, with very different personalities. The first group likes to move forward quickly and be morally conscientious but constantly splits the party. As a GM I try to guide them from place to place without the option to split except where it might be helpful. The second group sneaks around and is beholden to no moral code except self-interest. My job is to guide them through the adventure by subtly hinting at consequences.
This is by no means a complete guide, but it does outline the basics of having a good start. Remember that the goal is to have fun, but doing a little work ahead of time can maximize success at your first session.