I recently brought up the fact that I play D&D to a friend, and a few days later he came back to me and said that he wanted to play. I was excited at the idea and said that I would be willing to run a one-off session for him and some other friends. The first friend used to play somewhere around 15-20 years ago (certainly before 5e), but the other people have never played.
RPGs are fun to me because of the idea of being able to go anywhere and do anything. I don't want to make a group of people read the entire PHB before they play for the first time, especially before a one shot game. Even further than that, I will be making their character sheets for them so new players don't get bored by filling out sheets of paper.
Now I am presented with a challenge: I am new to GMing, and relatively new to RPGs in general. How do I explain the rules of D&D to new players? What should I explain before we play so that they understand that they can do whatever they want, but are still constrained by rules and rolls of the dice?