Our greenhouse was a modular kit from Santa Barbara Greenhouses. One day over 700 pounds of pre-cut and drilled redwood and a bunch of hardware were dropped off at the end of our driveway; it took about a week to build. We joke that it's the largest Ikea project we've ever seen!
Southern California is already warm, and we weren't interested in growing plants that require humid conditions (as they wouldn't survive in our garden), so an ordinary glass-enclosed greenhouse wouldn't do. Instead of glass, we use window screen to promote airflow and regulate temperature; we also have a large fan on a timer to help keep air moving during the hot days of summer. During the short winter days, we have grow lights on timers to help the seedlings along.
As much as possible, we declare the inside of the greenhouse to be a "pest free" zone, releasing beneficial insects inside a couple of times during the year. Even then, we seem to have more problems with aphids and the such inside the greenhouse than outside; whenever a plant comes under attack, we put it outside for a "vacation" where natural predators can clean them up before bringing them back inside.