Choose wood that either naturally inhibits decay, like redwood or cedar, or use pressure-treated wood if you're not building an aluminum greenhouse. Pressure treated wood contains chemicals that can damage aluminum. Get 4-by-6 inch boards as they are best for the foundation.
Select a building site that has a level surface and is free of weeds and debris and has good drainage.
Cut the 4-by-6 inch boards to the size dictated by the plan for the greenhouse you're building. These boards create the rectangle or square base the walls of the greenhouse sit upon.
Lay the boards on the ground using a level to ensure your greenhouse doesn't lean once it's fully constructed. Measure each diagonal to ensure the frame has 90-degree angles. Screw the boards together using lag screws that are 3 to 4 inches longer than the width of the board to ensure the boards are held together securely.
Lay landscaping fabric under the frame. Pick the frame up and re-level when necessary. It's important to use landscaping fabric because the material needs to be able to let water through while keeping weeds out. Cut the fabric to fit the frame.
Anchor the base to the ground using earth anchors. Follow the instructions on the package carefully so that you create a solid anchor from the greenhouse base to the ground.
Finish the foundation by filling the base with pea-sized gravel. Gravel works well because it allows the water to drain through the base. Once you've finished the base, continue with building the remainder of your greenhouse.