There are two distinct ways of standing when riding your board. Either with your right foot on the rear, REGULAR, or with your left foot on the rear, GOOFY. As with handedness, there are more people that are regular than goofy but there is no right or wrong way of riding.
Here's a quick test you can do if you are just starting out:
Have someone give you a slight push from the back and whichever foot you pull out first to avoid falling will probably be your rear foot. Start riding this way but if it does not feel comfortable, try the other way and see if that's better.