You would need to assess the nature of the land you want to build on. Is it loose soil, or are there trees in the land near where you want to build the shed? If there are, and depending how mature the trees are, it means there are likely to be tree roots which will need to be removed. You would need an excavator to clear the land anyway, and it is just one more level of preparation you will need to undertake.
It depends on the angle of the slope you are building into as well. Once you have cleared the site, you would need to remove any danger of the earth slipping back on to the shed, and causing damage. That might mean building a retaining wall of some kind. I would definitely think about having concrete foundations for this structure.
To get some more ideas, I would ask a builder to come and look at the sight, with a view to giving you a free quotation. Even if you do not intend to commit to have him build the shed for you, he will give you an expert opinion on what needs to be done, and that’s all you really need to know. He will also advise you on the materials and supports needed to ensure you have a solid and safe construction.