Good for you!

Well, many people will probably write novels so I'll give you the short version:
1) watch video of the fight first and know when to CD and when to tank swap (if there is)
2) boss position is key - know where you are moving and where the boss should be placed
3) COMMUNICATE with your co tank and healers about swaps/tankbusters/positioning
That's pretty much it. If you are just starting tanking in a raid, you may want to try a couple older ext primals to get the hang of it. Susano ext actually may be good as it has all the above points I illustrated. Good luck!