In years gone by your greenhouse would have been made of glass panels fitted into either a wooden or metal frame. This is still a fine way to construct a greenhouse, but it is labor intensive, and the final product is very heavy.
Carefully decide how big your greenhouse will be before buying any materials. Once you conceive a plan, decide how much three-year-rated UV-resistant greenhouse plastic is needed to cover the basic structure. For example, if you construct a 10-foot-by-32-foot greenhouse, which is a feasible size for a home project, you will need a 20-foot-by-50-foot piece of plastic. The material usually comes in a 20-foot-by-100-foot roll. This means you would have additional material to recover the structure when needed.
If you decide to have raised beds in the greenhouse, buy 2-inch-by-12-inch redwood planks to form a box on the ground as a base for the greenhouse. The number of planks needed depends on the size of the greenhouse. If you wish to form individual boxes in the greenhouse you will have to estimate how many planks you will need. Redwood has a high resistance to groundwater rot and is not chemically treated like pressure-treated stock.
Curved hoops make for an easier construction than squared edges. Three-quarter-inch PVC pipe can be filled with 1/2-inch rebar (iron rod) and then bent to the needed shape. You can actually make the ridge beam from the same PVC pipe using T-connectors to connect the side pieces, or you can decide to make the ridge beam from a spruce 2-foot-by-4-foot piece of wood. You will need the 2-by-4 for the end support and the door frame. You will also need 3/4-inch metal pipe bracket hangers to connect the PVC pipe to any wood.
If you live in an area where there are a lot of burrowing animals you may choose to put gopher wire around the greenhouse. Another option is to drive 6-inch pieces of rebar into the ground every six inches around the structure to keep away the wildlife.
If you decide to build small greenhouses (in the range of 6 feet by 8 feet), it is actually possible to move the whole structure with a small tractor. You can even move such structures around with the help of three other people--one person for each corner.
Not everyone is handy, but no one wants to spend a bundle on a contractor. Those who are less handy can buy a greenhouse kit. All the materials you need will be in the kit and cut to the correct size. Such kits also come with detailed plans to put the device together.