Foot placement is different for professionals as it tends to change with your personal style once you began to feel comfortable. But as for a beginer you should use the line that runs down the center of the board.

Your toes should be on one side of this line and your heels on the other. When you stand on your board you will automatically put one foot forward, this is your dominant foot. This foot is both directional and for balance. the back foot is your "root" most of your weight should be on this back leg.

Below is a video I found on Youtube that should help you understand a few of the concepts I have been explaining on this website. Sometimes its better to see difficult instructions in action.....

video courtesy of youtube.com