Official Overwatch: Player's Handbook

In general, Overwatch is a TEAM game. If you want to win, you have to play as a team. A lot of people often forget this and play very selfish. The overall theme in this guide will often be how to play as a team and utilize your teammates and exploit weaknesses in the enemy team.
Carrying a team can be done in Overwatch by some heroes to some degree, but even the best players will have a very hard time carrying their teammates to victory.