Quasi–raiding and PVP are two very different things. In WG, your team is called a “raid group” because it’s easier for the software to deal with stuff that way, but it is not really a raid in any other sense.
You can join any of the battlegrounds and do much the same as you’re doing now–stay with the group, get in there and mix it up when you find Horde. You can even crank up Immigrant Song so you’re " …to fight the Horde, singing and crying Valhalla I am coming". (I have a macro with those lyrics that I use in BGs sometimes).
I think the easiest next step for you would probably be Warsong Gulch. It’s 10 vs 10, capture the flag. I don’t really care for WSG, but some people like it. First team to capture the other flag 3 times wins. To capture, your team has to go into the Horde base, steal their flag, and run it back to your own base, and retain control of your flag at the same time.
Arathi Basin is 15 vs 15 I think, and it has 5 locations that must be captured and defended. Each location awards points to the team that controls it as the game progresses. First team to 1600 points wins.
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Eye of the Storm** is sorta a combination of Warsong Gulch and Arathi Basin. 4 locations to capture/defend, and a central flag to capture for bonus points. First team to 1600 (maybe 1500, not sure) wins.
Alterac Valley is my favorite of the older battlegrounds. It’s a large map, currently 40 v 40 players. Each side has a “king”, and if your king dies, you lose. There are also other NPC bosses to kill. You protect your bosses, kill the Horde bosses, kill Horde, destroy towers, and capture graveyards. Also, each side has a limited number of reinforcements. Having your own towers or npcs killed reduces your reinforcements, and when your team runs out, you lose.
Strand of the Ancients is one of the newer BGs. I like it a lot. You start out either attacking a fortress in a D-Day style beach assault, or defending the fortress. The team attacking first has a time limit in which to break into the fortress and destroy a relic. When the time is up or the relic is destroyed, the game restarts with the roles reversed. If the first attacking team succeeded, the second team has to do the same thing faster, or they lose. If the team attacking first fails to destroy the relic, the second team just has to destroy the relic within the time limit to win.
Isle of Conquest is sorta like a combination of Alterac Valley and Arathi Basin, with a hint of WG/Strand of Ancients in the mix. It’s a good sized island, and each side controls a fortress with a king. There are resource locations that can be captured and controlled for points and other advantages…vehicles of various types, even access to a blimp that circles the battle and can be used to bomb the other side or parachute into their fortress before the gates are destroyed. First team to kill the other king wins, or it can be decided on points, I think…not sure about that.
You are really not ready to try raid tanking yet. Before trying to tank for a raid, you need to learn how to tank for a group in dungeons. Since I don’t think you’ve done many dungeons yet, I’d recommend you start by queueing up as a DPS for regular (not heroic) 80 dungeons, or even some of the higher level 70 ones. SfG can prolly give you some basic warrior tanking tips. My warrior is under level 20, and only tanked Ragefire Chasm a couple of times more than a year ago.
You could try some raidng now, if you wanted. Easiest raid out there is VoA, which is available anytime Allies control WG. Again, que as DPS, not as a tank. You may get rejected from a few groups because your gear score is probably too low, but some groups may take you if they’re just filling out spots in a raid, knowing they have the firepower to down at least the Frost guy.
You may want to read up on the battlegrounds, VoA, or level 80 dungeons before trying them. You can watch a lot of the boss fights on YouTube to see how things go. Also WoW Wiki and similar sites have walkthroughs and strategies for pretty mcuh everything in the game.