To get the most out of your bike and yourself you'll want to use a pair of clipped-in pedals. For mountainbike the most common is the SPD (Shimano Pedalling Dynamics) with a small cleat on the underside of your shoe.
We also have plattform-pedals where you can use any shoe and don't need a typical cycling shoe. The plattform-pedals are more common on comuter bikes or a standard, everyday bike.