What do you intend to use your greenhouse for? It may seem a silly question but the answer helps to identify your requirements.
Perhaps you're keen to grow fresh tomatoes, cucumbers, peppers, lettuces and peaches; or you wish to get a head start on the vegetable growing season by bringing on seedlings ahead of the warmer weather; maybe you enjoy growing flowering pot plants and cut flowers; or perhaps you need somewhere to resuscitate house plants; you could have a prize collection of cacti that you wish to exhibit!
Knowing what you intend to use the greenhouse for helps to identify the level of humidity and temperature needed which, in turn, guides your decisions about the base (concrete, slabs, earth, strip foundation etc.), watering, ventilation and heating needs. Depending on the intended uses you may be wise to consider purchasing a greenhouse with a partition, and create two different environments within the one building.