Tips for Tank beginners?

Try to take all the damage you can during team fights

Focus on protecting teammates for the most part, it can also be beneficial if you try to lead the team to the objective instead of going away or trying to attack from the back, leave that to offense heroes. Other than that there's not much more you can do.

don't go ahead of your team and focus on protecting your allies

Prioritize staying alive. Dont let your shield die. Communicate with your teammates. You have to be aware of enemy cooldowns.

Focus on protecting teammates for the most part, it can also be beneficial if you try to lead the team to the objective instead of going away or trying to attack from the back, leave that to offense heroes. Other than that there's not much more you can do.

Like everyone else said, try and protect your team-mates while also maximizing your ultimate usage.