A great floor can be made from pressure treated 4 by 6's, stepping stones or slate, black plastic and pea gravel.
The wood frame will allow you to erect your greenhouse and the plastic will trap water spilled when watering the plants, which will then evaporate when the sun heats up the house. The gravel will give you a place to nestle the potted plants so that they do not fall over and will keep the mosquitoes from breeding in the spilt water. The plastic will seal the base from out side wind and stop the grass from growing inside your new greenhouse. The stepping stones will give you a better walking surface. And the wood base will make it easy to level up your greenhouse by shimming up between the frame and the ground. If you plan to grow plants in the ground use the plastic floor and some coffee cans with the bottoms and cut out, plant the cans half way in the ground through the plastic, plant the plants in the can, it works great and the coffee can gives you a convenient way to water the individual plants.
You will need pressure treated 4x6's. Cut the 4x6's to size and lay them flat on level ground to form the shape of your greenhouse so that they overlap and butt joints. Screw them all together, four screws at each corner. Then staple the plastic floor to the top side. The green house is installed on top of the frame at the outer edges. Install the floor as shown in the drawings. To anchor the house see drawings below.