this is taken from a professional shed company, perhaps your better off seeking their advice?
Shed Bases
Sheds need to be on a good foundation, though not necessarily as substantial a foundation as is sometimes thought. A large solid concrete slab will of course be excellent, but is a considerable amount of effort to mix and lay the concrete required for even a modest shed. If you decide on this route, then the slab should be at least 100mm - 4" thick. Generally the larger the shed and the more it used as somewhere to work - walking in and out regularly, the more substantial the base needs to be.
Wooden sheds always come with a wooden floor, usually consisting of tongue and groove boards held together with battens, these will come into contact with the ground and no matter how well soaked in preservative they are, they will rot if placed directly on the soil and so need something to keep them off the soil.
The purpose of a shed base is to provide a level platform for the shed so it can be erected properly and to keep the wood away from damp soil. If anything it is useful to have the shed base standing slightly proud - half to an inch will do - of the surrounding soil.
Perfectly adequate for the majority of purposes for simple storage sheds up to about 8ft x 6ft is a base of paving slabs laid dry onto levelled soil.
Mark out the area of the shed, strip off the turf if there is any and smooth out the the soil as well as you can.
Lay a string line along 2 edges of the shed and place the first slab in the corner, level it using a spirit level for most purposes, soil is perfectly adequate to level the slabs except if your shed is being placed somewhere fairly damp or perhaps low lying.
If you want a more stable and solid base or if the area is damp or low lying, a dry sand and cement mix of 8 parts builders sand to 1 part cement could be laid as a bed below the slabs to a depth of about 2 inches. This will also make it easier to level the area and to ensure the slabs are horizontal above it as the sand-cement is easier to move around than soil and settles less.
Lay the next slab against the long edge and level it with the first using the spirit level. Continue until the first row are all laid, then start on the second.
Put the wooden shed base on your slab base and build the shed. I'd always give the underside of the base a really good couple of coats of preservative before it goes down as you'll probably never see it again.
You can get by with less than a complete covering of slabs, as long as you have the full length of the battens that hold together the floor supported
