It is possible that the ground is OK just as it is. A good drainage system around the house should keep your floor dry. Some soils are very stable and drain well and an added base may be unnecessary. Remember that the finished floor needs to be higher than the outside ground level.
There's this great stuff you can get at the concrete supply place called road base. It's what's used under the roads for drainage and stabilizing. It is crushed rock in various sized pieces, usually 3/4 inch on down to powder (called "3/4 minus" in America).
Four inches (or more) of road base, dampened and tamped, make a very stable and very cheap base for your floor. Some people sift it and put down layers of the different sized particles that separated out, starting with the largest and layering up to the smallest sized particles.
You can use gravel (river or crushed) or shale for the base of your floor instead.
Remember to tamp it well. If you live where the ground water level is high, you may want to make an extra thick (10 inches or so) base or a layer of large gravel and a layer of road base. The gravel will discourage water from wicking up into the floor. If you use a thick base for your floor, it is important to make your ceilings that much higher too.
A leveled layer of dampened, tamped, sandy or silty soil, two or three inches thick, is sometimes used as a base or as an added layer on top of the gravel or road base layer. This needs to have enough clay in it to bind itself together when it's dry. If you are making a floor of flat stones and bricks, the sandy or silty soil makes a good bedding for them.
You can put pipes under the floor to heat your house. Hot water circulates through the coils of pipe, heating up the floor and the interior space. I have never done a floor with heating coils in it, but there is information available on putting heating pipes into concrete slabs. You can modify this information using natural materials. If you want to bury pipes under the floor, you can put them in this soil layer, but only if you are not adding a screen layer that will insulate the heating pipes from the interior.
Some natural builders put a 3 or 4 inch layer of tamped light clay/straw (see page 143 for instructions on making light clay/straw) over the gravel before putting down the soil. This prevents the soil from sifting into the spaces between the chunks of gravel and adds a little layer of insulation. The light clay/straw is tamped well or compressed when you tamp the soil layer. It will crush down to an inch or less. Loose straw or old sheets or newspaper would serve the same screening purpose, like on top of the drainage ditch. If you are troweling a cob floor right onto the base, you won't need to worry about it filling in the air spaces between the gravel.
Other possibilities for an insulating base layer are to mix clay slip with vermiculite, perlite, or pumice.