Anything you can move freely in, made from fabric that handles sweat. Most clubs are relaxed about it, though a few still ask for predominantly white or for a collared shirt, so it is worth checking before a first visit. Court shoes matter more than the clothing does.

Yes. Tennis apparel works across all the racquet sports. The requirements are the same: freedom of movement, fabric that handles moisture, and a pocket that will hold a ball.

A skort has built in shorts underneath, usually with ball pockets. A plain skirt does not. Most modern court skirts are made as skorts, which is why the two words get used interchangeably.

Close enough that it does not flap on the serve, loose enough that it does not restrict the shoulder. Lean towards your normal size rather than sizing down, since court clothing is cut with movement already built in.