Every system or style of play takes time to learn; when to press, when to drop off. When to overlap, when to underlap. Patterns of attacking play, ways to build up.
You also need to get used to your teammates. Who talks and who is quiet? Does he want the ball into feet in that situation, or does he want it in behind? Does he respond well to criticism, or praise? Does he want the ball in tight situations?
Your manager and coaching staff need to get used to you. Right/wrong things to say, good/bad in certain positions or game states, actual ability, ability to deal with pressure.