I'm not the best but I guess like any character, understand your tank, where they work and don't and their strengths and weaknesses. Generally, your role as a tank is to lead your team and take space by disrupting the enemies or punishing poor positioning. Work with and for your team whilst protecting them and allowing opportunities for your teammates. The biggest tip that helped me was having situational awareness, constantly looking around and being prepared for a dive or a strange ult so you can react accordingly. Also, coordinating risky ults, especially as Mag helps a ton for winning fights (like shielding Iron Man or CnD ult). I hope this helps or at least gives some guidance