Mario's Tennis is a tennis game for the Virtual Boy, and is a single player experience. It was published by Nintendo.
It should be easy to find Mario's Tennis for the Virtual Boy available for purchase. In good condition, the game is worth about $2.00. Mario's Tennis was released on 8/14/1995.
At the Title screen, press L, L, R, L, R, Select.
Beat the game on singles 5 times with Mario, Luigi, and Donkey Kong.
Throw the ball up with the A button, wait two seconds, then slam it with B.