In a proper riding stance, your forward foot should be just behind or on the front truck screws, and your back foot should be near the back of your board, near or on the tail. This is the most stable and safe riding stance, and having your back foot already on the tail makes a quick kickturn easier.
This is the most difficult thing to learn when you're first getting started, but the good news is you'll be riding successfully when you get it figured out! No fear!