Basically you can do it one of two ways. Either go and join some Rp's and get to know some people before you host your own or just go ahead and type up an interest check and dive right in. For me, over the decades, I have always found it easier to join a few Rp's first and let people see how I Rp before actually starting my own. It lets you build some "cred" with other Rpers and you have a better over all chance of getting an RP to take off once you host it yourself. Like it or not, there are little "clicks" on the forum and people have their ways. So meeting people first tends to work better over all.