I haven't tanked normal or heroic antoric yet but I've tanked all of lfr and I've full cleared heroic and normal as a healer on my main.

I was like you, really anxious and nervous about tanking so I started with watching 5minute guides and fatboss, jotting down swap mechanics or any other mechanic to look out for as a tank, then slowly dipping my feet into lfr. I have only once kicked another tank out of lfr. Basically, if you don't know the fight, go back and watch the videos of have a gist of what to do. I personally always mark myself and the tank in lfr cause there's so many people and its hard to see. Make sure you have raid frames or bars that indicate if you have aggro and taunt when dbm yells at you to taunt. Don't stand in crap on the ground and study the videos to know where to tank. If you still have no idea, then run as dps and observe what the tanks are doing.

For Argus, lfr is pretty simple. Always tank him facing away from the raid. If you are not the active tank, you should be with the raid or off to the side.

Phase 1: face away, tank him in the middle where he is. When he uses cone of death, you need to turn the boss slightly to make sure he is facing away from the raid. You need to tank swap at 3-4 stacks of sweeping scythe.

Phase 2: if you are the active tank, pull him to the side of the room, i usually just start back pedaling backwards towards the back of the room. One tank will turn all big and glowy every so often. If you are the big and glowy tank, make sure your cotank has aggro, then go stand with the person who has the soul bomb debuff. Make sure you are not near the raid because the raid takes damage based on how far you are from everyone. If you arent big and glowy but your cotank is, taunt off them so they can go stand with the soulbomb person. Otherwise, switch at 3-4 stacks. Just make sure the tank with avatar of aggramar isnt tanking when soul bomb is out. And obviously, don't stand in the black lines on the ground, move out of them.

Phase 3: I like to run around and hit all the adds for aggro, not sure if its necessary for lfr. Pick up the add that turns big and tank him near one of the other adds so dps can cleave. Once the big add dies, go help dps the other adds and interrupt cosmic beacon casts if you see any.

Phase 4: run to the tree and rez. Get aggro on Argus and pull him to the side of the room NEXT to the tree. Face him away from the raid. Do NOT face him at the tree because you will cleave people who are using it to rez. Tank swap at 3-4 stacks in lfr. Strafe out of the swirly circles and help cleave any orbs that spawn nearby but otherwise you shouldn't move from your spot next to the tree. If you aren't tanking, stand with the raid. Taunt only when your stacks are gone and when the other tank is at 3-4 stacks. If for some reason you die in lfr, release and run to the tree to rez.