Drywall by itself is not sturdy enough to stay flat. It will conform to whatever surface you place it on. You need to get a flat surface to place the drywall on. The best advice I can offer is get the longest straightedge you can. I have one that is 48" long (picked it up at a paint and wallpaper store). Use it to check how flat a surface such as a hollow core door. Lay it on it's edge on the surface and you will see any bulges or dips in the surface you are checking.

Hope this helps

Ken