1. Watching a guide to get a quick overview. True learning begins if you raid it yourself since your group has to find the way that works best for you (like Lambdafish already wrote).
2. Your gear has to be good enough to keep hate from your dps and that you can take the hits from bosses and mechanics without getting a oneshot. For A5S should the 220 Gear do the trick.
3. The most important part is that you have to be prepared to wipe a lot during the learning phase and even after that (someone will always mess up) and keep calm, raging people are useless in every raid.

I started to tank our raids because i was sick of tanks who can't even use their cooldowns properly or hold hate. On top of it i was just tired to play NIN in raids.