Suggestion on writing an SUS: Open up a text file or a word document and type the first things that come to mind for a while and then see what you've got, and then make those things into something coherent. If writing long paragraphs doesn't come naturally to you, you have to force yourself to write long paragraphs until you get used to it.
All you have to do is search those roleplays until you find one that interest you, and then, if you see that the roleplay is accepting, then you would have to follow and paste a guide line - a Sign Up Sheet. (which I reccomend you do it if they have posted an OOC - Out of Character thread at the Roleplay Lounge).
Try to follow the requirements as best you can, and then, if they accept you, you can start roleplaying! Usually they'll post a IC thread - which is the actual thread that you would roleplay in, at the Roleplay Corner.
When you start roleplaying, just remember what they ask you to do as you post (like specific events or tasks they could ask you to do) and you should be good to go from there. :3
Oh noooo not at all! The mentor might recommend/suggest you a few threads or join groups, but anything is all up to you. No need to ask permission with your mentor. His job is to teach you how the RP corner works (and teach you how to RP if needed). :)