There is a tank guild 101 by Hoku
Two main things that a tank needs to do:
- CC an out of position target and has your teammate follow up for the kill. The target could be an enemy squishy who places himself/herself as a frontline warrior (you often see them in Silver but it become more rare into higher rank), an enemy diver who dive into your backline (make they pay for threatening your backline) or the enemy tank who is (a little) too far from his/her team.
- protect your backline by CC the enemy diver. You shouldn’t place yourself too far away from your backline. Therefore you can protect them and the healer can heal you.
Diving the backline is usually the bruiser’s job and it should be done after the tank (either side) engages.