The most important part of any base preparation is choosing the most suitable area in your garden. Firstly consider what you want to use your building for, this may alter the position it will eventually be located. If an existing concrete, paved or decking area doesn’t exist, an ideal site would be a fairly level dry lawn or compacted soil.
As long as the ground is relatively level (within 6-9 inches) and not subject to bogging the building could easily be assembled onto bearers, bricks and blocks. Alternatively you can choose the more permanent approach of a reinforced concrete base, which is ideal if the ground needs building up or lays a little wet.
Something else we would always recommend, whether you are choosing a concrete base or not, is pressure treated floor bearers. These will raise the floor of the building off the ground allowing air to circulate, thus keeping the underside of the floor dry and extending the life expectancy of the building, timber doesn't mind getting wet but it doesn't like staying wet.