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MLPchan isn't the ~busiest~ of role play locations, but it's decent. Each canon (or little RP group) has its own rules, so you'll want to peek into a thread that looks interesting and ask about rules.
In general, you can be a canon character or an OC. Depending on the canon, you can also be a human/martian/kryptonian/whatever. OCs are accepted, as long as they aren't Mary Sues.
So… yeah. Pop into one of the existing threads (Open Equestria and Better Layton Never seem to be the most active), ask how you join, and go from there.