Are your feet flat?
There are two ways to tell what kind of feet you have; you can either just look at your foot (this is less accurate) or you can look at your footprint (make a footprint by running on paper with wet feet).
If you have a flat foot, your footprint will look like a foot shaped blob. There will be almost no inward curve from your big toe to your heel; there may even be an outward curve. If you are just examining your foot, you'll notice that as you press your hand down on the bottom of your foot that your foot flattens out to conform to this new surface.
If you have a flat foot you are probably an overpronator. This is the most common of the three types of feet. All this means is that when you run your feet roll inwards. You will probably need a shoe that will maintain stability for you. Look for the keywords "motion control" and "stability" when shopping for running shoes.
