While not entirely mandatory, I thoroughly recommend each learner (or in the case of siblings, each household) have a Player's Handbook for D&D 5th Edition. It will be invaluable for knowledge of the game. If learners would like to broaden their character customization options, they can also look into other source books from Wizards of the Coast (the makers of D&D), such as Xanathar's Guide to Everything, Volo's Guide to Monsters, and/or Mordenkainen's Tome of Foes. These are far more optional.

Learners will not need to use any apps or websites beyond the standard Outschool tools.

Dungeons and Dragons games vary widely on content, and can go from zero mature content to vastly mature. While running any games on Outschool, I will be staying far from all mature stuff. It does involve violence, as D&D is known for combat as well as role-playing, and sometimes I might describe a scene that would be fairly grotesque, but I will also adapt to the players, so if anyone is uncomfortable, I can easily tone that down. I have found that most players enjoy the more realistic descriptions, however.