Drive through the landscape avoiding the trees/rocks. Hold the vampire balls within your lightbeam to destroy them, the closer you are to the vampireball the faster it loses its shields, though be aware if the vampire ball is not within the lightbeam it will rapidly repair its shields. Van Helsing loses a small amount of energy (denoted on the left side of the screen) with every collision also the faster you travel the more energy is used, once all the energy is gone its GameOver Van Helsing.
You can see which direction the balls are in by the dots on the outside of the screen, the larger the dot the closer the ball.
Unless you change it in the options the mouse steers you and the mouse left & right buttons accelerate/decelerate.