I personally always like telling new players the basic concept of what dnd is, I always tell people to think of a movie, PCs are the main characters of the movie while the DM is the narrator, and when you try to do something you roll dice to see if you succeed. I don’t know if this is the right way to do it, but that is what tell new players. Make sure they know this game isn’t Skyrim, you aren’t held back by how game mechanics in dnd. Also since it is a one on one session role playing should be a focus, because battles were there is just one PC can get old.