Well, there's a few ways you can do that. Well, if you have an idea as for what you want to do, but don't have a partner, I would consider making an Interest Check. There are a number of forums for different topics (fandom, fantasy, modern, etc.) for the general gist of your RP. You can make a thread that explains your idea, and then people will comment if they're interested.
You can post it here: https://www.rpnation.com/forums/looking-for-partners.67/
So for interest check's you'll need to specify what type it is (fandom, fantasy, etc.), and also just put a tag on it (which is like for categorizing the post).
Okay I found a group. I posted my oc now I'm waiting to be accepted (btw idk what to look for when I get accepted) I want to find out where the rp in the group is
well, you posted in the CS probably. Once accepted, the GM probably has an embedded link to the IC (in character area) and the OOC (out of character area)