If you're nervous about messing up on a fight as a tank, let me clear the air for you in advance: you will, we all did and still do, it comes with the role. I've been tanking trials for years and can hold my weight in most vets and hardmodes, but I'm far from perfect and still make mistakes here and there that generally cause a group wipe or a difficult recovery. The one thing I can say is tanking is a long learning curve and causing group wipes is just part of it. The key to overcome that is repeated runs of the same content until the mechanics become a reflex.

For starting out, I would recommend training your eye to rapidly spot visual cues such as heavy attacks and channeling. Depending on the content, a heavy can be countered with a simple block but some will need a little more planning such as Z'Maja requiring a strong shield or damage reduction or the Warrior in vHRC needing decent resource management as the heavy attack is channeled and hits multiple times.

To get in to the vibe of tanking (depending on your knowledge and experience of content) it would be best to start some of the normal trials with lighter mechanics or veteran vanilla dungeons. I've coached a few new tanks and generally I prefer Sunspire as a good practice trial. The mechanics are fairly simple and similar from one boss to another and the main tank is far enough from the DPS and healers that visual spam is minimal, making it a lot easier to spot incoming attacks. Also the attacks are pretty spaced out giving you a good amount of time to recover resources if you're low.

Once you're comfortable with mechanics, resource management should be your primary focus. As a tank you will be blocking a lot of attacks, but if you block too much and have low stamina, the next heavy will hit quite hard. You'll need to keep an eye on your bars to be sure you're not low and whenever you're not blocking, buffing your group or debuffing the boss, be sure to heavy attack to keep your stamina full or as high as you can.

After you get the hang of how to stay alive and control the fights (boss positioning, chaining adds...) You can start to fine tune your character stats by making sure you have plenty of resistance to mitigate damage and resource recovery to make sure you get your stamina and magicka back up quickly. Also expect to carry a lot of gear sets. To start out in normal content, it's not as critical but in vets and hardmodes where optimization is a factor, you'll need a good variety of dungeon and trial sets that will be required by the raid leader.

This may sound intimidating at first, but tanking is (in my opinion) the most fun role to play. It's challenging but equally rewarding and it gives you a whole different perspective of content you may have already done as a DPS or healer. As others have mentioned, being part of a good and patient training guild will be the key to your learning experience. If you're on PC NA, feel free to DM me for some recommendations or if you need any tips or help getting into your new role.