Kind of odd your FC buddy wants you to pick up WAR specifically, especially for OTing. Not that they can't do it, or DRK can't MT, just weird because I think which tank you choose should kind of be up to you. Even more so if you're just starting out.
Anywho, it can be a lot to take in at first, but it's not too bad once you're in the water. When it comes to tanking 101, there is a quote I always like to bring up:
Applied to tanking, when you look at an experienced tank mow through a dungeon, it seems simple. But when you break down their mechanics it suddenly becomes complicated. But after you get down and apply what you've learned, it becomes quite simple all over again."Before I learned the art, a punch was just a punch, and a kick, just a kick.
After I learned the art, a punch was no longer a punch, a kick, no longer a kick.
Now that I understand the art, a punch is just a punch and a kick is just a kick."
-- Bruce Lee
This is my favorite tanking guide: https://www.youtube.com/watch?v=0fS-FVrY74Y
Definitely start with dungeons, and do lots of them. Practice is key. The leveling stages are brutal, but important. All the experienced tanks have been through it. Some players will be patient with you. Others not so much. Never, and I do mean never pull wall to wall in a dungeon because you feel pressured to. You have to crawl before you can walk, and you have to walk before you can run.
Trash pulls in dungeons are one of the most threatening things you'll face while tanking. Managing the damage coming from a boss becomes much easier after learning how to manage the damage coming from 6+ mobs all at once. I do highly recommend Hall of Novice, and also the three 4-man trials to give you a bit of a taste of full on 8-man trials. In 8-man you won't have to worry too much about tank swapping outside of EX and savage. If you are OT though, always be prepared to take the reigns should the MT drop.
Good luck out there!