I just bought an 8X12 wooded shed kit for my backyard (including floor kit). I will not be using concrete, unless this is recommended? My problem is that I have a very unlevel yard. A local stone company recommended stone to level it, is this a good idea? Also where I want to put it is a low point in my lawn where sometimes we rain water settles. Any suggestions here?

Yes, you can raise the shed over the wet area using pier and joisting with an 1/2" OSB flooring. This way, rock fill will not be necessary and the shed will be high above any wet soil. Make sure the concrete and piers are set into the hardpan soil before installing your shed post and floor underpinnings. Using a water level will give adequate floor joisting reference points for level posting heights. Nm

Well, 1/2" osb flooring is not very suitable for a shed floor alone. to use this alone with joist would lead to the floor sagging. also, plywood would be much safer if only one layer was to be used, then i would suggest using 3/4" and having the joist on 12" centers instead of 16" centers. The best thing to do would be to use 16" centers on the joist, then use two layers of 3/4" flooring. This could be done with 3/4" decking tounge and groove, then put 30# felt paper down, then put the 3/4" plywood down. This would give a good stable floor with an additional vapor barrier, and would not sag. As far as the water setting situation, if you do not pour footings for piers made of concrete blocks to support the floor at a safe height above ground level you WILL have settling issues that will lead to the shed becoming unlevel, and adding additional stress to the structure. You do not need to pour an entire perimeter footing, just pour some footings for the piers.

Just like mowerman says,

Use 3/4" 4x8 T&G Marine Ply with 2 x 12 Joists at 12"oc and you can park your car or a Sherman Tank in the shed. Yeh, also pour a perimeter 8" stemwall foundation to keep the water out.
Or you can save bucks and let the floor sag a little for a shed that will have less than a sunshine load on it. This is California sheds that last 50 years. I did not mention that an
8 ft span 2x8 DF joist at 12" oc is good enough to support an ATV. I have one and the
1/2" OSB holds fine, its the joist span sizing that will sag if there is no midspan support.

So sizing depends on what you plan to do with the shed. If it is going to be used for a cabin then I agree, use at least 5/8 T&G flooring if over 12" joist centers. Pier and posting is all that is needed for a girder system on hardpan poured pier pads. For a shed use a perimeter rim joist belly band in shear with posts to piers with a midspan girder on posts to piers. This is basic pole house structure design that works in high water table areas with posting going deep enough to hardpan.

No sense in spending a ton of money just to keep a lawn mower parked in a shed. If this is being done under permit, then this is a whole different ball game. I won't go there. Good Luck, nm