There are two basic approaches. One is to have a hardstanding of slabs, these should be laid on sand so that they can be levelled. The shed is then sat on these slabs.
The alternative is to have a beam all round the edge, this can be either preserved timber eg sleepers, or brick or blocks. Some hardcore or sand underneath for drainage will help but is not essential. The base wants to be level before you start to erect the shed.
Both methods are used extensively, I don't have a preference.