I haven't roleplayed in yearsss so dunno where or how people RP online these days and I'm sure there's different ways to RP, the way I used to do it with my friends was pretty simple. We would basically just chat via messenger/skype/dA messages etc, but writing as if we were our own OCs talking to eachother in a specific scenario and writing their actions between asterisks.
If you're RPing this way, you just need to know the basic information of your character before starting to RP, like their appearence, how old they are, their basic personality (if they're shy, stubborn, friendly, etc). And as things start happening you have to think about how they'd react to certain situations and develop more of the personalities on the go, many of my OCs got more defined personalities while I was RPing like this with them!