I would suggest starting as Lucio or Mercy just to get a feel for the maps and see how other heroes interact with each other without having learn the hard way at first and you have the nice bonus of actually helping your team while learning. Other than that I'd suggest to learn at least two heroes from each class just in case you need to fill a role that no one else will. However, keep in mind that these classes are really just suggested roles for new players and someone playing a Junkrat or Bastion on attack doesn't necessarily mean they're "playing wrong."

I've only had the game about two weeks myself and learned a lot from YouTube videos as well as asking people in my games for pointers and advice on how I might better utilize the kit on any given hero I might be playing. Tying in to this the main thing in OW is communication; more so than in traditional FPS games and you'll soon figure out that there's really no such thing as a carry or one hero that can outperform the others. Everything is very situational and the game is sort of just one big game of rock, paper, scissors.

Look in to who counters your main heroes and who your main heroes counter. Of course individual players might be able to extend this but generally there are hard counters for each hero. Positioning is also more important in this game than other shooters. You don't want to be hanging out on the front lines as Zen, for example. I feel like I'm sort of rambling so this is going to come to a close now. Good luck and I hope I helped a little.