Because I've only played extensively on Tunare server, I can really only provide information about blue servers, but the priests of discord were(I'm assuming still are) used to toggle your character from non PvP to PvP.
At one time everyone started the game with an item called a tome of discord. If you handed in the tome to a priest of discord you would be flagged as a PvP. Now they've done away with the tome, but I'm sure since the priests are still around, that you can still be flagged as PvP. Try hailing the priests, that's usually a good place to start if you want information on quests the NPC gives.
If you're on a blue server(non PvP), I would strongly suggest you do NOT become flagged PvP, unless you're just looking for a challenge for the sake of being able to say you did it.
You live under some pretty strict limitations as a PvP on a blue server, and unless they've changed it lately, the PvP flag cannot be removed.
You can of course still duel even if you're not flagged PvP on a blue server, so there really isn't much point to being PvP(I swear they need a shoulder shrugging smiley
).