Mine was a strange one. I wasn't a newbie, but I hadn't posted once since Guildfall (tl;dr a little while back the site that was originally RPG went down for good and everyone had to migrate to this one, I'm not sure of the actual details because I had been on hiatus for RPG before it). So, it showed that I had been on forever, but hadn't posted once. When I finally put up an interest check that made some people go "huh, that's weird," but others were actually interested in the idea. Met some new friends, got some old friends from other sites to come here, etc.
Long story short, I guess my idea from this is don't be intimidated by the fact you don't know anyone here yet. I didn't know anyone here when I started my "first" (on this technical site) roleplay, as all of my friends from RPG before Guildfall have pretty much disappeared. I brought just one friend here and she had to make a new account entirely, but she didn't help me advertise so the unfortunate bastards I've drawn were all my own doing.
If you got an idea, especially one you've been sitting on, go ahead and make an interest check. Get the idea out there for people to see, stake your claim to it before someone else makes something eerily similar completely by coincidence. You may have to be a little patient since no one knows you so you aren't being actively stalked; it took almost a month for me to get enough people for me to be comfortable with starting my roleplay, but we've been going on for over three-fourths of a year since then.
If the idea doesn't get taken too initially, then that's when I'd say you start going to other roleplays, for a few reasons. One is the reason why everyone has said to do so, being the-making-of-the-friends, but another reason is because you're here because you want to write, right? That's what I assume at least, so write, get better at it, and improve your own idea while you wait. Once you've made yourself a good friend group, then put up the roleplay again, and I'm sure it'll get eaten up in a good way.
Basically, if you're impatient like me go ahead and do it, but if you actually have patience then do what everyone else is saying because it's probably less risky.
I should be working right now. This is what I do instead.