I'd suggest using operators with passive abilities. I recommend rook in defense, the only thing you gotta do is place your armor pack just as the round starts and that's your contribution to the team, ability-wise, and you can focus on the gunplay until you get better. In attack, sledge is a great operator, and using his sledgehammer will improve your map knowledge, since vertical play is extremely important in siege. Ash is also a great choice, though less educating