In all of the major towns theres these little machines next to the ones that look like gumball machines. Just click them and then pick one of the arena's (1st is more popular usually) and hit the button on the bottom left hand corner.
Do you get exp for these pvp battles? If so, more than regular mob kills? (sorry for asking in someone else's thread, but reading about pvp basically made me curious as to whether its worth it or not) Also, is it categorized on level or something?