Use the mouse to move the paddle
Left click to serve while you're touching the ball (you can move the paddle to curve)
Hit the ball with your paddle. It will head towards the other player and will bounce off any walls it hits.
There are three bonuses you can get: Accuracy, Curve, and Super Curve.
Curve the ball by moving the paddle just as the ball hits it. Get accuracy by hitting the ball on the little square in the center of the paddle.