You've got a lot of considerations when building a base for your greenhouse. A good base must provide drainage, be anchored to the ground and prevent weeds from growing on the greenhouse floor. An inexpensive yet high-quality choice for greenhouse base material is wood, especially woods that resist decay like cypress or cedar.
Plan the size of the greenhouse. Lay string out along the area you intend to build the greenhouse. Measure the perimeter with a measuring tape to determine how long your wood should be.
Cut 2-by-4-foot-long boards to fit the frame of your greenhouse. Place the boards out along the intended layout.
Screw the frame together, using lag screws at least 5 inches long. Use one screw for each joint, for a total of four screws. Insert the screw into the centre of the board and use an electric or manual screwdriver to insert the screw.
Cut a length of ground cover or weed barrier to use as flooring. Ensure the ground cover is long enough to fit under the frame, with enough room to staple it to the frame. Do not use a black tarp, since this will not allow water drainage. Ground cover is breathable, so water can drain through it. Ground cover also prohibits weed growth; hence, its nickname of "weed barrier."
Attach the ground cover to the wooden base with a staple gun, using one staple every 3 to 4 inches. Pour at least 2 inches of gravel or crushed rock on top of the ground cover. Construct your greenhouse frame on top of this base.
Make sure your planned site is level. If it isn't, you'll have drainage problems later on.