Greenhouses are usually supplied with a plinth of the same material they are constructed with and this can be placed straight on to the earth or concrete where the house is to be erected, or, preferably, put on to a foundation and dwarf wall of bricks or concrete.
It is really a matter of personal preference whether you decide to concrete or pave the whole floor or leave borders down one or either side. The soil in borders can eventually become disease ridden and will require sterilizing, or preferably changing if you are growing the same crops year after year, but if you are growing conservatory-type permanent ornamental plants, these can be planted safely into the border soil. The alternative is to use pots or growing bags, replacing the compost annually.
It is often recommended that the best position for siting a greenhouse is running east—west, which admits most light and provides both a sunny and a shady side, but in many gardens the ideal aspect is not possible, so aim for an open, sheltered, level position away from objects which cast a heavy shade, and especially not under trees. Having said this, even a north-facing lean-to can be put to good use if no other aspect is available, though the choice of plants is more limited. If possible, the greenhouse should be accessible to the nearest water supply and preferably the electricity mains as well
If you are erecting your own greenhouse, do take notice of the instructions supplied with it and follow the manufacturer’s recommendations and the sequence of construction — this is usually done for a reason; not only does it make the assembly easier, but if you try to short-cut or use your own methods you can often end up with a bent frame and a pile of broken glass.