We're starting on our fall garden too. We've never done one before but we did greenhouse last winter and grew tons of lettuce, peas, carrots, and spinach. The trick we found with greenhousing was growing stuff that doesn't need a lot of light level otherwise you have to supplement with light fixtures. Our fall garden is broccoli, cabbage, corn, squash, tomatoes, beans, and lettuce. The quinoa, sorghum, peppers, and sweet potatoes are still growing from spring so they'll get harvested too. Then later we'll plant brussel sprouts, carrots, rutabagas, parsnips, kale, lettuce, and spinach which can all grow into the cold weather of late fall/early winter.
We built our own greenhouse out of pvc pipe and heavy mil plastic sheets. It worked great over the raised beds we already had. If you have a much of snow load you would have to watch how steep you make the roof otherwise it will collapse under the weight. But it is a very economical way to go. Ours is 12 x 18 and we only spent about $50 total.