it really comes down to wha you want to do, if you want a melee character and a healer (which is what it looks like to me) then i would have your melee as your main and the healer you can just heal with, rogue/druid would keep you more mobile than a rogue/priest combo because all of the HoTs a druid has, but again it's all up to you really, just do what you enjoy and if you dont like it, just make 2 new ones.

And no there is no specific "how to 2 box here" mini site, but you can use the search up there at the top and search for 2 boxing or dual boxing or some such.

and you will be boxing 5 before long lol, i started out with a lock/priest and took me all of a month to take the plunge into 5, now i'm doin 10