As a budding do-it-yourself greenhouse gardener, you can buy a premade kit and put it up yourself, but it's really not that much harder to build your own greenhouse from scratch. And it only costs about half what you would pay for a kit. A greenhouse is essentially a frame made from PVC pipe, chain-link fence tubing, wood and rebar, which is then covered with plastic to let the sunlight in and retain warmth.
Regardless of the material you choose to frame the greenhouse, the basic approach will be the same. The website Frugal Living Freedom recommends two-inch PVC (plastic plumber's pipe) because it is perhaps the easiest for a beginner to work with. You can get PVC, fittings, pipe glue and plastic at your nearest home improvement store or plumber's supply shop.
Calculate the dimensions of your greenhouse beforehand in order to know how much material to buy. The PVC pipe functions as a skeleton holding up the skin (layers of plastic). Most greenhouse designs are rectangular, with a pitched roof and doors at either end. Remember to make it large and tall enough that you can walk comfortably between the rows of plant beds.
Your first order of business is to build the rib sections. Each requires four lengths of pipe, one for each wall and two for the pitched roof. Each complete rib is spaced approximately five feet apart. Install horizontal pipes the length of the greenhouse as needed for stability at the juncture of sides and roof, as well as where the ribs meet the ground. You can use wood planks and drywall screws to attach it to the PVC frame if necessary for added stability.
Wrap the entire structure with UV-protected greenhouse film. Where the film touches the ground, leave enough extra material to staple it to a board, which helps keep the film pulled tight and also serves as an anchor. Drill holes through the wood and drive rebar a few feet into the ground to add enough support for windy days. If you're still concerned about stability, use ropes and stakes, as with a tent, to stabilise the structure.
You may also choose to equip your greenhouse with exhaust fans, overhead irrigation, lighting, heat, and maybe even allow the wall film to roll up during nice weather. Install raised plant beds inside and your greenhouse is ready for year-round growing. Other rigid materials for the frame will work fine also.