One question I get fairly frequently is therapists (and others) reaching out to me and asking how to get into D&D, with the explicit purpose of running D&D games for their clients. And in turn, I've seen a lot of people responding to this question with not so great advice, nearly all of it focusing on teaching them the rules.
To learn what D&D is all about, just watch The Gamers. It's about 45 minutes long, and it will tell you pretty much what D&D is at it's core in an entertaining way. To learn the basic rules, pick up a players handbook. If you're having questions about the rules, that's completely normal, and bickering about unclear rules is one of the favorite pasttimes of gamers. I still have a lot of things that aren't clear to me about the rules, but being able to quickly google it, or more often, ask my clients to help me figure it out (or come up with a house rule on the fly), has worked out fine. Overall D&D is more accessible and easier to learn than it has ever been.
But when learning D&D for any sort of applied RPG work, it's not the rules that are important. It's the implementation- not the what, but the how.
Very often, I see people recommending therapists play X or Y module with their clients, because it's easy to run these modules. Too often, no attention is paid to the question of "What is your client going to get out of this, and how?"
It's not just therapeutic and applied D&D that suffers from this, either. I've seen enough cases of therapists using Minecraft with clients who absolutely no plan, and often minimal familiarity with it. It's one thing to use it to create a space to build rapport, but with such a powerful tool, it seems absolutely wasteful of the game's potential and your client's time to be just playing it, and unintentional use risks invalidating these tools, as there are plenty of doubters that will be quick to say, "What, are you just wasting time playing videogames with them? How is this therapy?"
So, the first most important thing for a therapist to do is to review what the specific goals of any D&D/TTRPG intervention will be, and build from there. Once they have their goal in mind, THEN you start learning the rules. D&D has a ridiculous amount of rules, splat books, house rules and eratta that can take a lifetime to learn across the editions. Instead, by having a clear idea of what's important, you can both set the expectations of the group ahead of time, and study what matters.
Say you want to work on a group to teach teamwork and frustration tolerance, and feel like a dungeon crawl that requires teamwork and the ability to lose characters and have to make new ones might be a good way to do that. Then, study up on dungeon design. Perhaps you want to focus on building social skills- There's all kinds of social heavy tools you can use to add plenty of great role playing opportunities, or tricky puzzles that require group strategizing. Or maybe you want to focus on teens building their voice and doing identity formation- There are tools to help players build really unique characters and tie them to the plot and each other. No matter what you want, there's tools out there you can use, and in my experience D&D players love to share ideas. And if you want more formal training, there are plenty of resources out there, including a number of Geek Therapy courses, and Game to Grow. But ultimately, what it comes down to is being absolutely intentional about what your goals are, and working from that spot.
Hopefully this is helpful for people just getting into the therapeutic/applied RPG field. I'd love to hear your thoughts, please reach out to me @rollforkindness on twitter!