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To protect your plywood sheets from warping, the best way to store them is by laying them flat. Moisture can seep up through the earth, causing your plywood to rot. You can use any size lumber you have, as long as it creates a frame that is big enough to contain the plywood. Space your lumber about 2 feet (0.61m) apart to ensure it's supported enough that it doesn't sag. If you want to keep your plywood laying flat but you don't have enough room on the floor, you can install a rack on the ceiling of your garage. This will keep the plywood out of your way while still protecting it from warping. You can purchase a hanging storage storage rack from a hardware store for around $100 USD, or you can design and build a custom rack if you prefer. Most ceiling beams can safely support about 160 pounds (73kg). This means you can safely store 3 sheets of 12in (1.3cm) thick plywood, or 2 sheets if they are 58in (1.6cm) thick.
Section off a 4 by 8ft (1.2 by 2.4m) section of your garage floor. Use lumber to create a base that will keep the plywood off the ground. Store your plywood overhead if you don't have the floor space.