A racket. So you don't have to hit the tennis ball with your hands.

Tennis balls. The fuzzier, the better.

Court shoes. These come in a variety of styles and price ranges: expensive to very expensive.

Tennis swag. Traditional tennis swag is preserved at Wimbledon (although it has been tweaked ever so slightly), which requires its competitors to wear white shirts, white shorts/skirts, white shoes, white socks, and no small amount of white sweatbands.

A court. With a functioning net. Robert Frost once wrote that "writing free verse is like playing tennis with the net down." He's a smart guy, so he's probably onto something. All we know is that the net is definitely recommended when it comes to playing actual tennis.