okay, lets see if i can be of any help!

roleplaying, whether in forums or one on one is exactly the same, so here's the basics to it~

you have to treat it like a story, because that's what roleplaying is, essentially; a story you co-write with another person, or several.

so you have to pretty much choose the genre and characters you wanna go for. my advice would be to get a rough draft of the plot, but don't develop it too much; one of the funnest things in roleplaying is that you never know what the other roleplayer is going to reply, what twist the stories are going to have. that element of surprise adds a nice touch to it. c:

other than that, there are a few different ways to roleplay, like script

Me: *punches you in the face* Unacceptable.

and paragraph

Enraged, I delivered a punch to their face. "Unacceptable." I snarled.

regarding actual forum roleplaying, whoever starts better give everyone else a basic idea of what they have in mind for the RP, in order to avoid any confusion, and preferably add the first post, to give it a good feel. 

also no god modding, OP characters, Mary Sues / Gary Stus and so on.

all in all, it's just having fun and getting to know people. C: