Before putting your shed together you need to ensure that you have a strong, level foundation to put it on. With smaller sheds if you have a hard flat area, such as a well laid patio you may get away with putting it straight onto that. But by and large it is better to prepare a specific proper foundation. The traditional method is a concrete or paving slab base. If you are using concrete then prepare it several days before constructing the shed so that it dries out properly.
The use of specifically designed interlocking plastic bases is becoming increasingly popular for shed bases.