Having a good team composition and doing our job in that ?
During the match make sure to observe who on the enemy team is causing you the biggest problems. Is the enemy having a Pharah and you do not have a hitscan ? You will see Pharah taking out a lot of your teammates and someone needs to change. Bastion grouped up with healers and shields ? Make sure to change into a team composition that can take it out. A Widowmaker that is doing a lot of the killing ? Whatever it is, you have to figure it out as fast as possible and make sure to change into something that can counter. A team with healers is more likely to win. Your team needs to take out the enemy healers (for example using flankers), that should focus the enemy healers and take them out. This will make it more likely that your team will eventualy win the match. Last, you can have a good team composition, but if you are not doing your job the team will still fail.
Don’t overextend ! Stick with your team. Work like a group and make sure that when you take out an enemy (making them a smaller group) that you fight mostly instantly. Also make sure that if a teammate gets killed before the fight starts, that you retreat and wait for him to return. You can peak and shoot a little at the enemy but make sure that you do NOT die ! Your team will then have to wait again and you will have given them ult charge.
Ulting wisely ?
Make sure that when you use your ult, you preferably use it together with your teammates. During team selection you can allready think about who you are going to combine with. However, do not hold on to your ult for half the game to do this. The perfect moment for your ult will likely never come. Some ults charge a lot faster than others and you might allready have your second ult where your teammate only has his first. Don’t use to many ults per teamfight. (3 max) Make sure to recognize that your ult is not adding value anymore and you are better of holding on to it for another fight.
Using the map in our advantage ?
Try to use high ground as much as possible. If you can play just as well on highground as on low ground, you should be on high ground ! Don’t stand in front of a chokepoint but behind it. The enemy will have to go through a tight point. All these advantages make it easier for you to shoot your enemy and more difficult for them to shoot you.
Some things you can commucate to your teammates: Ult charge and when you are going to use your ult and who you want to combine with. Positioning of you and your enemy’s. Target focus. Killing an enemy together is a lot easier than alone. Call out missing abilities. If you see an enemy using his ability (for instance, tracers E) call this out so your teammates know what to expect from that enemy.
Everything is ofcourse situational, so nothing in here applies to every situation, but in general these questions should be something that you can atleast think about and will help you improve !