Don't start with D&D. Start simpler and work up.
1.) Tell stories with them. Go through a story like Goldilocks that they know and ask them what Goldilocks does at each of the pivotal moments in the tale. Then tell it again, but give her some other choices - Does she drink from the giant cup that is too big, too small or just right? Then tell the story with her again, but add in the trip to the cottage... Each time you make the story more expansive with new challenges. No dice. No damage. Whatever they attempt, works.
2.) Then add a simple rules mechanics system to add a chance to fail things. I like the Dread system for the simplicity: Set up a Jenga tower. Whenever a player needs to have their character try something, have them pull a piece from the tower. If it falls, they fail. I modify it when 'training' to roleplay by having both the DM and the player pull from the tower and whoever pulls that piece that fells the tower 'loses' the encounter at the climax of the story.
3.) Basic D&D is now ready to be played. Simplify the rules and handle all the math for them so that all they need to do is tell a story and roll a d20. They should roll to hit, roll saves, roll initiatives, etc.... but you should tell them what they need to roll to succeed before they roll. Then, as you play, explain the math that is going on 'behind the scenes' and ask them if they want to start to roll damage, etc...
There are also some great roleplaying light games, like Mice and Mystics or Betrayal at House on the Hill, that can give some idea of being a character.