Building a greenhouse. What is best kind of heat to use?

My husband passed in June 2017 before we finished the inside of our retirement home we had started building together and he had started a greenhouse for me also. There is a 8'x12'concrete pad and 6 --4x4 posts set. They are starting to twist. We were planning on using clear and smoked fiberglass with wood section around the bottom of the greenhouse. I don't know how to get started and what would be the best way to build it and the best and cheapest heat to use in it. Thanks. Brenda
q building a greenhouse what is best kind of heat to use
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