I would like to construct a small green house for raising only annuals in. I probably only need 200-400 square feet. Especially if I can build two levels for the plants. Just looking for some advice. I know nothing about greenhouses or nursery raising. I can think of climate control and irrigation as essentials. What else am I looking at in getting this thing going? Would you recommend anything specific for the walls/ceiling of the thing? Plastic? Canvas? I really have no idea where to start.