Although the lifetime sheds include a floor, the manufacturer recommends placing the shed on a flat structure such as a wooden foundation made of treated 2x4's and OSB board that you can pick up from your local home improvement store.
The instructions (available to view online) show just how to build it and what materials are needed. The other recommendation is a concrete slab. The concrete slab is more pricey to erect. It is very important to make sure it is flat and drains well. If it isn't flat you may run into issues with your doors closing properly. Feel free to contact us with any other questions.