Tankadins are drawn to PVP, and to the Arena specifically for the Merciless Gladiator's Gavel.
What I'll share here are some of the lessons I learned about Prot PVP during my grind. Like anything else, PVP takes time and practice to learn to be effective. Learn what classes you do well against (Enhance Shaman, Rogues, Fury Warriors, Cat Druids), and what classes give you trouble (Casters). Learn the "tells" that give away a spec. A Warrior dual wielding is Prot or Fury. A mage with an Ice Shield is Frost, and won't be able to POM/Pyro you. A Paladin with a 2her is Ret, a Warrior with a 2her is Arms. Hunter's and Pets that go all big and red are Beast Mastery. Dual Wielding or 2her wearing Shaman is Enhancement.
I went the route of being a PVP Healer. It was easier to gear up for two roles (PVE Tank / PVP Healer) as I already was building a Healer set for those times when I wouldn't be tanking.
I played a 5v5 team and a 3v3 team. The 5v5 team was a 3 dps/2 healer matrix. The 3v3 was a 2 healer / 1 dps matrix.
In both cases the other healer was a Resto Druid who would start each match out in Cat Form and stealthed. The idea was for the other team to target me as the healer of the group, then my Druid could heal fairly unmolested. My main job was to keep our healer alive, and the healer would keep the rest of the team going.
One of the first questions you have to answer for yourself when you consider Prot PVP is are you willing to sacrifice some talent points to make it easier to PVP. The single talent that makes the most difference in PVP is Spiritual Focus, followed by Improved Concentration Aura. What you want to achieve is 90 to 100% uninterruptable healing. 4/5 in Spiritual Focus, and 3/3 in Improved Concentration Aura gives you this.
The other option open to you is respecing each week. It will cost about 200g to go from Prot to Holy and back to Prot (50g each respec, plus the gold in training the talented spells like Holy Shock, Holy Shield, etc.) On a moderate team, you should be getting about 300 points a week, which makes this a 10 to 12 week grind. Would you have paid 2000g to 3000g for the Mace outright?
Blessing of Freedom on your Warrior, especially if the other team has a Hunter or Frost Mage. Blessing of Sacrifice on the other healer, or one of your DPSers. Just make sure it's somebody who will be taking damage. Sacrifice will break many CC effects on your. If the other team is being diligent in purging off Sacrifice from your target, you can try to "hide" it on a pet. Blessing of Wisdom on yourself, and Blessing of Light on anybody you will be healing. Kings for anyone else, but I found Light is better than Kings over the course of the match.
If you have a Druid on your team, call for an innervate at 40% mana.
If they put a pet on you, seal and judge Light, Seal Wisdom and start whacking on it. If they put a pet on your other healer or on a squishy DPSer, taunt it off. Taunt is not specifically considered CC, so it works even on big red pets. Some hunters are pretty good at redirecting their pet to the primary target but even they do that, they've lost precious DPS time while the pet runs towads you and then gets redirected to the target.
If the other healer or a squishy dps is being focused fired, hit them with Blessing of Protection.
You wouldn't walk into Karazhan in Greens or SSC in Blues and expect to do well would you? Don't walk into Arenas undergeared with delusions of grandeur either. But don't let that stop you from doing Arenas. It's going to take some time to get enough points for the Gavel, the sooner you start earning points, the sooner you can pick it up. If you don't have a team, you need to get one today. Do your 10 games and get some points this week.
You need stamina, Resilience, healing, and mana per 5. Resilience is analogous to defense. It helps reduce the damage you take in PVP. One of the first items you are going to want to get is the Insignia of the Alliance. Then get it upgraded quick to the Medallion of the Alliance. Crowd Control is everything in PVP, you need these to get you out of crowd control and back into the fight. In order to get this you will have to do Battlegrounds.
Battlegrounds also give you PVP epics for your Rings, Back, Belts, Boots, and Bracers.
Also consider spending Arena points on your Arena Gloves and Shoulders. That's the quickest way to get the 2 piece bonus of 35 Resilience. As of 2.3, Season 1 gear will also be available from Battlegrounds. You could build quite a nice PVP set just from Battlegrounds.
There are couple of resilience drops in PVE dungeons like the Mithril Bark Cloak from Laj in Botanica. You are going to be killing him many times for your Righteous Shoulders, so pick this up.
Protection Paladins can be very useful in most Battlegrounds.
In Warsong Gulch, put on your Heroic (high Stam) gear and run flags. You can provide your own Blessing of Freedom, Cleanse yourself, have high hit points, high armor, and Ardent Defender. In 2.3, you can also pick up the new Pursuit of Justice which will be very nice for running flags.
In Arathi Basin, there are few things as powerful as a Protection Paladin guarding a Flag. If you bubble is up, that guarantees the other faction isn't capping for at least 12 seconds. If you get feared away from the flag, Avenger's Shield can stop up to 3 people from capping.
I made a macro for when I'm defending.
/bg Incoming attackers at Mine, Send help if you want to hold the point
Replace Mine with whatever point you are holding.
I use a very similar strategy in Eye of the Storm, guarding points and calling for help.
Alterac Valley will probably be the easiest Battleground for a Tankadin as there are actually mobs in there with aggro tables. If you PVE gear is good you can tank multiple Warmasters, and our fast threat allows DPS go to all out on Drek / Van.