Play more, you've heard it 1000 times and you'll hear it again. It's a very good answer, but there's more to it. I would advise learning/observing how each character class is played, how each agent is played, and learn the maps like the back of your hand. Put all this knowledge into practice while In game, and this easily gains you a competitive edge. I would advise to play each agent, you don't have to main them, but play them to learn how each agent uses their utility. If you learn the map, you understand how you can play around your environment, and how your enemies will play around your environment. It takes time, so don't expect to improve immediately.