So, roleplaying... it really really depends which side of the spectrum you want to get in on.
On the one side, there's what amounts to collaborative storytelling - doesn't need much setup, the easiest way is to grab any of the forum platforms that have a working subaccounts mod (just for convenience) and you're basically good to go from a technical standpoint. On the other, there's the whole situation of whether you're doing full blown table-top grade RPG, as in you have a games master/dungeon master who is the primary storyteller and the other players have to not only react to events and choose what they're going to do next but then get into dice rolling to see if they were successful. There aren't many platforms that are good at doing this because this needs dice rolling at a minimum but also frequently you want some kind of character sheet tracking to make it easy to get at the stats.
There are, of course, blends of everything in between as to what you're looking to do, and this also manifests in terms of likely community dynamic, attitude, player personalities etc.
Then we have the real question you pitched: how do you get started with the roleplaying? Well, you need things to happen in the form of plots, you need a place for them to happen in and you need a backdrop or a setting to give it all context.
There are several ways to go about this, each with their own pros and cons.
First up we have the 'fandom' style, this is more popular over in the collaborative storytelling side, where you have a fandom people like (or at least know) so the rules of the world and some typical locations are set up, as well as key events in that world's history are known, and then you pick a place or time in that space to do something. I, for example, had many hours writing in various eras of Harry Potter-based forums, whether that was post-books, or 1950s (when certain key characters are just revving up) or even the 1970s (when little is documented and it was possible to invent whole swathes of new story that could slot in). The obvious pro is that chunks of worldbuilding are done for you, the con is that people inevitably want to play as 'actual' characters from the story aka canon characters which can lead to drama over who gets the avatars and who gets to play, followed by accusations of staff bias etc. There are also lots of debates and arguments about 'being too true to the material' vs straying too far, especially for fandoms that have powered characters (e.g. pretty much anything Marvel or DC, Potter, anything that has regular and special characters because people inevitably want to be the special characters)
Then we have panfandoms. These tend to be much more 'free for all' where you can bring along whatever characters you have and they'll probably fit. Want to do The World's Finest double-tagging with Iron Man and Cap? Sure, why not? Much more loose in terms of rules, much more chaotic but can be a lot of fun if you're not too bothered about having going with the flow. Also gets around some of the aforementioned drama because you can have multiple instances of Captain America or whatever, each from a different slice of their universe. It can be as loose as you want - but you probably want some rules to keep it *sane* because superpower characters inevitably powercreep on each other over time.
Then there's the panfandom-adjacent segments, things that could have been a fandom but are more generic. For example, the supernatural themed sites with spooky events in the woods with vampires and werewolves that could have been any one of a dozen Young Adults fiction series. Magical Universities are another, as are the Completely Made Up Superheroes That Aren't Just Knock-Offs Of DC Or Marvel Honest. Basically imagine any sizeable fan-followed property then imagine a budget knockoff version from Wish.com, and what that might look like. Pro: it's not tied to a property, so you don't get any drama if you want to do Magical University but avoid all the JK Rowling debate, but the con that comes with it is that people have to do some actual *reading*.
Then there's the real life crowd. Imagine a place that's a stylised, idealised version of a place. The small town in mid-west America that has a distinctly atypical population distribution. Also connects with the 'slice of life' crowd, where the overall vibe is just a place to hang out, chill, low-key drama. The main setup for all of these is, of course, a place that's your own place to call home.
Then, there's the wholly original. I've seen everything from 'original historical' set in a pseudo-Victorian era to something approaching the Ancient Greeks, to future cyberpunk in a demon-infested San Francisco. The limit here is simply your imagination. Pick a scenario that sounds interesting, build out a bit of world to give it structure. The headache is that you will need to do a bit of building, but that it will often go somewhat ignored. As a rule, the more worldbuilding you do will be interesting, but the less people will inevitably read it. So you will inevitably be spending time explaining the lore, answering questions about the lore.
There is one last category: the faciltiation site - the kind of site that isn't 'the site is the game' but the site is merely a facilitator for many kinds of games. In that particular space, sites like
https://www.rpnation.com/ are already pretty established. Takes less effort to be a generic RP home hub, easier to get going but the depth of storytelling is likely shallower - there will be a bigger focus on one-on-one conversations-as-storytelling. If going to enter in this arena, you'd need to identify what you can do differently to something like RP Nation to make it viable. It's not that you can't compete - but you need some draw to bring in other blood.
Getting going on the writing - I'm sure there's any number of resources on the actual mechanics of worldbuilding and storytelling. The trick is to be broad and open and not too prescriptive about things that don't matter too much, and that's true for all the different setups.
As far as finding people, that sort of depends on what style of site you set up and who your target demographic is, but chances are this is going to involve site-to-site advertising, affiliate linking, listing on one or more of the directory sites, one or more of the directory Discords, maybe on Reddit, maybe on Tumblr. If you want to talk a bit more about the kind of material you're looking at, I can suggest places you can probably advertise it at.
What I do know is that building an RP site on the side of an existing site is that it's a distraction of focus that doesn't help - the facilitation site/hub model works but even then it's its own site dedicated to RP, while all of the others are single-site-for-site-stories. That is the only way I've seen RP work at all out of literally thousands of RP sites I've now looked into. Even the facilitatation hub is a tiny fraction compared to the rest.
Hopefully that makes sense, but feel free to throw any questions out there - we can always put them out for wider discussion if they're of a general nature and interest to other people who might be interested in the RP scene.
