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).
Although solid, these polycarbonate materials bend so can cover a curved, hoop frame. Or they can be cut and used in sheets for straight walls. These materials usually last for at least 15 to 20 years before they begin to yellow and reduce the light coming through. Such glazings made specially for greenhouses, compared to those that may look similar from home stores, often are treated on the outside to resist the UV rays from sunlight, and treated on the inside to reduce condensation that can build up and drip in humid greenhouses.
Of course if you want a more decorative greenhouse, such as attached to a home or building ("lean to" greenhouse), you may consider glass. This is harder to construct and deal with, can break, and unless twin wall (similar to energy-efficient windows for homes) lets more heat out. Glass greenhouses tend to be more expensive, but maintained can last for decades. If you're handy with tools and building, you may want to construct your own home greenhouse. Otherwise you may want to consider just buying a "kit" with all you'll need. You can then buy accessories such as heaters and benches. Some kits even include these. If you'll be growing vegetables in the ground you may not even need benches. There are many suppliers of greenhouse kits online and in garden supply catalogs, even from some seed companies. These can be located with an internet search for "home greenhouses" or similar.
If buying a home greenhouse kit, some other considerations in addition to glazing type are where it is coming from--is it suitable for a northern climate, and what is the freight charge? You might even call the company and, through talking with them, see if they can provide answers to any questions, and judge their customer service (if you have technical questions once your greenhouse arrives it helps to have such expertise handy). They should be able to give you tips on the foundation needed for your greenhouse (often this is just wood anchored in the ground), and how to make your greenhouse more energy efficient if you'll use it during colder months.
Unless growing just in April in May in the north, you'll need a more powerful heater. Make sure the greenhouse supplier can recommend both type of fuel heater (common is propane), and appropriate size. If you'll be growing during colder months, on windy days, you'll need a much larger heater. It is better to spend a little more now and make sure you have enough heat if needed. For the seasonal, tent types, an electric space heater may be all that is needed.
On the flip side is ventilation. Larger greenhouses have automatic vents and fans that run with electricity (although they consume very little). For smaller greenhouses consider automatic vents that open and close on their own, just by expansion in a rod, with no power. Invariably the greenhouse will need vents open and closed, with the sun coming and going, to keep from getting too hot or cold. Even if you're at home all the time, this can become quite an issue.
The main consideration when buying a home greenhouse is the size. Just like rooms at home, you can usually fill any available space and wish you had more. Buy the largest greenhouse you can afford, and have space for. When planning its location, allow space either for an addition or another one nearby. Make sure to locate near your home (if not attached), near water and power, in full sun, and easily accessible for moving plants and supplies.