Welcome to the Shard! We're glad your here!
There's no need to give the rp a rating, we have a specific rating for the entire rp forum outlined in this thread. Pretty much everything is already pg-13 (ish), so no need to worry about things getting more intense. You could outline if you wanted the rp's content to be cleaner (less violence, etc.), if you wanted too.
Rp's also generally need something to kick them off and get things started, even if it's just a simple character introduction.