The first thing you want to do is go to a training course to find what character (s) you may want to use.
Then what you want to do is play your first game of quick play. You may be the best because you tried out the characters before most of the people.
The next thing to learn about are the best assault classes. The first assault class that I would use is soldier 76. The reason I use him is because he's very powerful and can heal himself.
The next assault character I would use would be Mccree. I chose Mccree because in some cases is you get 2 shots you can get an instant kill. You can also stun and then shoot all the bullets at once.
In my personal opinion Hanzo is one of the best defense heroes. His arrows are extremely powerful and his ultimate is almost an insta kill
Bastion is also a good defense. He turns into a turret that is very powerful. He is also weak but can heal himself. His ultimate is also literally him turning into a tank.
My last defense character is junkrat. He fires high damage explosives. And his ultimate is a tire that explodes and kills everyone nearby.
I believe that almost all the heavies would be a good choice and they are equally good.
Now to the healers.Zenyatta is a robot monk that has a red tracking device. He throughs his levitating balls as an attack and he would probably be your best choice if you want to both fight and heal.
Mercy would also be a very effective healer because of how fast she can heal. Mercy would be the healer to use if you want to stay behind lines and heal. Her ultimate also brings people back to life.
I'm sorry if I didn't talk about any of the other characters. This is just what I've found useful. Please leave a comment if you think I should change any thing.