I advise doing some planning instead of making inaccurate judgments. Decide on how big of a grow area you want, and estimate how much soil and how many edge-stones you will need. Keep in mind that the plants you will be growing may not like the dirt that your greenhouse is built on. Therefore, if growing trees be sure to clear out about 3-4+ feet deep of dirt to allow room for the deep growing roots. Annuals and most low-growing perennials should be fine with as little as 1-2 feet deep of soil.
Picking a soil:
Digging and filling:
Again, keep in mind of what kinds of plants you will be growing in the ground bed and dig to an appropriate depth. Upon digging your bed, be sure to add a couple inches of gravel or rocks. This will provide drainage to the soil above. Some gardeners like to add a meshed material to place on top of the gravel layer. This helps preserve the drainage of your gravel by preventing soil from leeching. It is not required, but it will help in the long-run. Once your drainage layer is done, start filling the ditch with soil.
Place edge-stones around the perimeter of the ground bed to prevent soil dispersion and to define the grow space. You may also choose to fill soil all the way to the top of the edge-stones making a sort of raised bed.
Trouble-shooting ground beds:
If the soil has become compacted overtime it may inhibit the absorption of water and suffocate roots. To fix this problem you may add soil amendments which will recondition the soil and improve its quality. Alternatively you may till the soil using a shovel. This method involves the physical break up of the compacted soil returning it to a normal state.