Cook whatever it is that you already know how to cook best that requires direct heat, e.g. steak, burgers, chicken, pork chops, port tenderloin etc. Build a two zone fire because that is one of the main things that a Weber Kettle allows you to do well. Then go cook. That way you can concentrate on learning the grill vs. learning how to operate and control the grill and how to cook that particular item at the same time.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill