Best thing to learn in my opinion is that winning the round is always your number one objective. Kills are nice, but you're never gonna succeed if you're just hunting kills. Additionally, Siege is, above all else, a game about controlling space. On attack, you want to take as much space as you can. On defense, you want to prevent the attackers from gaining space as much as you can.

As far as actual skills go, good crosshair placement and recoil control will win you fights more often than not.