Ranked is only for max level characters.
Role is determined by your characters current specialization in the Skill Tree.
You can join the Warzone queue to participate in them both.
Thanks very much for taking the time to respond, and for the informative update. I was a bit cautious about Arenas at first but I'm looking forward to them the more I hear about them.
I have to say that it's a little worrying about the specialisation. A guildie of mine is one of the best Guardian Tanks I know but his current spec is 22/23/1 which would presumably put him in as a DPS. It would be great if people did not have to give up specs that they've decided suit their natural playstyle to conform to the matchmaking system.