A basement just is not livable until it has a smooth concrete slab for a floor. The moisture from the ground will always be coming up and the air will smell like a dirt floor. By pouring a concrete floor, you almost eliminate the moisture problems and get a storage space in short areas or a living area in 7-foot or higher basements.
Level out the dirt on the basement floor as much as possible with a shovel and a garden rake. Scrape off the surface of high areas and fill in the low areas. Work right up to the edge of the walls for a straight surface.
Bring in 2 to 4 inches of gravel or sand to top off the dirt, forming a surface that it easier to level. Check with your local building codes to see what the requirements are for your area. Drag a 2-by-4 over the layer to level it out.
Cover the aggregate with a sheet of the 6mm or thicker polyethylene to form a moisture barrier between the ground and the concrete slab. The sheets must overlap by 6 inches at all the seams and at the edges.
Space the No. 4 reinforcement bars every 18 inches across the floor to add extra rigidity to the slab and reduce the chance of cracking over time.