Now reaching rank of main tank in your guild will not happen over night, it takes a ton of hard work. You must learn every fight in every level 70+ instance and raid in the game. Not just the boss fights but also how to approach every mob in that instance/raid. Addons like Deadly Boss Mods and sites like Bosskillers.com and Tankspot.com will become your best friends. But I suggest you tank it, tank it early and often. Start tanking as soon as possible any instance you can get into even BC or vanilla WoW, go for it even if your high level for it, just do it. Since every time you tank is different, you will take something different away every time and learn from your or your groups mistakes. I look at every wipe as a learning process, try it a little different and see if the outcome will be better. You don't need to do anything else, because they will notice your confidence and even more importantly, your brains. When the time is right, you will be rewarded for your hard work. But that's all part of the fun, being the leader, the one your group or guild depends on to know the fights and your knowledge of game mechanics will make everything else easier the next time after a beautiful wipe.
Being the main tank isn't the easiest thing to do. In a perfect world you will be able to maintain your rotation without a hitch, will never lose aggro and never single-handedly cause the death of 4, 9 or 24 others because you missed a button. But we all know that's never the case, many factors have to be taken into account. Its your job to position the mob, maintain aggro while keeping an eye out for adds, looking to see who is creeping up on threat and be ready to use either a Hand of Salvation or Righteous Defense/Hand of Reckoning to maintain your aggro lead or taunt the mobs back to you. All the while you keep an eye on your healer's mana bar, because when she starts getting low, you have to weave some of your cooldowns in your rotation like Divine Protection or pop a trinket to ease up the damage your taking or know when to use the wonderful Lay on Hands when times get really rough. Its a rare occasion when its truly the healers fault since you have so many ways to stay alive, but an undergeared healer will make things tough. Then comes the next aspect of being the main tank, keeping your mouth shut through wipes and trust me, there will be wipes. You will see repair bills never imaginable, my highest has been 86 gold, but its ok, its part of the job. You sticking it out through think and thin is how you earn the respect of your guildmates. Once they get the fights down, its all smooth sailing and the there is nothing better in all of the World of Warcraft then defeating a boss with millions upon millions of HP together as a cohesive group working as one. Its the best feeling, makes all the hard work worth it.
I am proud to be the main tank for my wonderful guild, The Dark End Tavern. I joined my guild about a year ago when I was level 58 or 59 during a run I was tanking in BRD. I was guildless at the time since my previous guild became a “hardcore raiding guild” and I had no idea what that even meant. So I asked one of the guys in the group if I can join and the rest is history. I have been lucky in my picks of guilds, only been in a few and they were all great. The Dark End Tavern welcomed me and I haven't looked back since. So the best advice I can give when looking for a guild is to find the one that feels like it just fits. You will spend more time with these people then you would your real life friends. You want them to be something more then just a group of people you play a game with, but more like a group of people that you would be friends with in real life.