Building a foundation for your greenhouse offers you many choices.
Some people, especially if they buy a pre-made greenhouse, will simply lay it on top of whatever ground exists at the location they choose. That may be gravel, or dirt or even grass. Some pre-made greenhouses come complete not only with roof and walls, but a floor, as well.
If you choose to build your own greenhouse, you’ll probably want to prepare the ground underneath it in some way. This could be as simple as smoothing the earth and laying down railroad ties spaced an inch apart. An alternative step up could be creating a simple platform of eight-foot 2 x 4’s or 1 x 6’s nailed to 4 x 4’s spaced out about every 18 inches under the slats. That way you have a solid base and drainage.