Entry into PvP matches can be started through a quest from your Grand Company NPC. The Wolves Den itself can be accessed through a ferry found in Moraby Drydocks.
After you get to Wolves Den and enable it on your duty finder, all you have to do is queue up! At patch 2.1, there are only 4v4 matches available on one map. Each match has a 2 minute prep time at the start and the entire match lasts for 30 minutes or until one team is wiped out.
Participating in PvP matches give you PvP experience and Wolves marks. If you earn enough PvP experience you will level up your PvP rank and gain one action point which you can spend on skills or traits specific to PvP. These skills and traits can only be used during PvP matches. You must slot these skills like how you would slot a cross class skill.
It takes 3 Action Points to get one PvP skill and 1 Action Point for a PvP trait. Most of the PvP skills and traits have 3 levels each and increasing a skill or traits rank increases different things, debuff durations, potency increase or cooldown reductions.
Wolf Marks is the currency of PvP. For every PvP match you participate in (win, lose or draw) you gain a certain number of Wolf Marks. You use these marks to purchase PvP gear and PvP materia. The PvP gear itself has a new stat called Morale. This stat replaces your Magic and Physical defense against human players and also increases your damage dealt to them.