How do I cook food in this square stone oven?
It's a barbecue. The firebricks in the bottom can be used to contain the charcoal in a smaller region, and/or be stacked to raise the grill. I recommend raising the grill as going by the picture, it looks like the food would be too close to the coals and burn before it cooks through. 

The chimney takes the smoke away from you quite nicely and is well worth having.