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.
But those simple designs leave you with the problem of controlling weeds, replacing floorboards, controlling mildew and other issues associated with wooden floors.
The next step up is laying a cement foundation, similar to the type under many houses. This is easier than it sounds, but it requires more effort than the other alternatives and has a few potential drawbacks.
Creating a cement foundation requires laying out an area, smoothing it and building a temporary 'container' around the area you intend to pour. You'll want to make it fairly smooth and level - not an easy thing to do unless you have a fair amount of experience pouring cement.
In the end, you'll be left with a semi-permanent foundation which would be difficult to move or remove later if you change your mind about the location.
But a cement foundation will last longer and give you some options about heating. You could lay carpet or tile on top with heating tubes or wires underneath, for example. It's easier to take care of and very sturdy. Cement floors also can absorb and reflect a lot of heat, which can be handy in a greenhouse.
Building a good cement foundation will require a couple of weekends. The materials, tools and construction plans are available at a hardware store or can be ordered online and delivered.
Beyond following the directions for mixing cement, creating the frame and properly forming the surface, the key is temperature and humidity. It's important that you carry out the project when you expect moderate to warm temperatures, relatively low humidity and no rain.
Whichever option you choose, make sure you plan ahead. Make sure the floor is sturdy enough to support the benches, tables, pots and people that will be inside. After you have all that installed, it's much tougher to replace the flooring.