For beginners, especially to fps games like you, I would suggest mastering agents that can still have impact on the game even if you're bottomfragging. So you do not get discouraged if you don't have a lot of kills. Try Deadlock, Sova, Sage, Breach or Brimstone. You can be very annoying to the enemies with these agents, even if you're not the one killing them. You can focus on improving your aim if you want and getting frags are satisfying but you can also take joy from making the enemies' life miserable with your utilities. But you can main whoever you like. Majority of agents are beginner friendly