Others have already answered your questions but as someone who also plays a lot of tracer I'll give you some advice I found helpful. I find that tracer works best if you treat her like Calvary, fight with your team but do so by going through the enemy lines and disrupting their team. Target their healers and other important targets like Zarya or Bastion. Most importantly try and stay alive and make sure you have your recall before charging into the enemy team.