Starter Rules (5e Variant Rule)
From D&D Wiki
This sourcebook is not for adding on to d&d, but rather it is for simplifying d&d for beginners. First of all, here is what is not kept or changed but simply removed: attacks of opportunity, flanking. Actions to remove: dash, disengage, dodge, help, hide, ready, search. Classes to remove: barbarian, bard, druid,monk, paladin, ranger, sorcerer and warlock. Races to remove: dragonborn, gnome, half-elf, halfling, half orc, tiefling. No homebrew (besides this). No backgrounds, feats, archetypes or multi classing.
I would recommend for the first adventure a simple, cliche, dungeon crawl with one player (that player being the beginner).