Wii Sports Tennis is a great way to play tennis when you can't get onto the courts.
Serve by flicking the controller up and then down, or press the A button to toss. Timing the hit at the apex of the toss may result in a power serve. To do this more effectively, use a slice shot (high to low). Another way that you can hit a power shot, is to flick your wrists as your serving. During the game, lob by moving the remote from low to high. Swing horizontally for a regular topspin shot, and swing from high to low for a slice shot (which may often hit the net).