The #1 factor in determining your rank is simply whether you win or lose. You could have the game of your life with 40 kills, 37 objective kills, 10,000 healing, 5 minutes objective time and 100% kill participation. But if you lose the match, you're gonna lose rank. Sounds like a shit deal, but there will also be matches where you couldn't land a shot to save your life, yet still won because every other factor somehow fell in your team's favor. This may sound like a volatile system, but because the only constant factor in your matches is
you, it eventually all levels out over the course of several dozen/scores of matches. There will be win streaks where you seem unstoppable and losing streaks where you want to quit the game, but, big picture, it all levels out in the long run.
Most of my improvement came from watching YouTubers. I was absolute garbage at the game when I started, so I began looking up character guides and went from there. Here's a screencap of my subscriptions, all OW related.
That's a big list to try to jump right into, so if you only have time for a few, I'd say the most useful ones are OneAmongstMany (particularly his VOD reviews), Skyline, MindFuzz and iostux. Aside from that, I also recommend the Overwatch University subreddit.
And yes, I'm on XB1. GT: Dinotective. Feel free to add me.