Starting your own flowers and vegetables at home from seeds lets you have many varieties you might not find otherwise at stores, can save you money, and is fun. If you are thinking of starting more than a few packs of seedlings, or already did last year and ran out of room to grow them on prior to planting outside, you may want to think about buying or making a home greenhouse.
First, ask if you need a small greenhouse or some other structure? What are you intending to grow? If you are starting seedlings indoors under lights, perhaps all you need is a coldframe instead to harden them off before planting out. If growing vegetables, perhaps you'll just need some low plastic tunnels over the rows. Yet most gardeners, if starting more than a few flats of seedlings, will find a home greenhouse useful, fun, and a welcome setting in early spring.
Home greenhouses come in all sizes, starting with small pop-up tents just for spring use (although I've seen them last fine in central and southern New England year round). These can be about 4 to 6 feet wide by 6 to 8 feet long, and about 6 feet or so high. For just a couple hundred dollars you'll be in business growing in an hour or so. Other greenhouses you leave up year round, especially in colder climates. A bit more sturdy and long lasting are those covered with plastic film. Larger greenhouses, similar to those used by growers, have a small fan inflating another layer of plastic on the outside. This creates an air space between layers for extra insulation. Such air-inflated houses usually are more than a home grower wants or needs, and harder to construct, with recovering needed every 3 to 4 years.
I prefer, and have, a small greenhouse made of a polycarbonate solid material. Unless just growing during April and May, you may want to get one that is "twin wall", having two layers with an air space between. In cross section they look like honeycomb. Even better for insulation, but more money, are the triple wall glazings (the word for greenhouse coverings).