Most 3D printers require you to have an STL (STereoLithography) file to print. You can design your object in any 3D modeling software, such as AutoCAD or any of the free software programs shown below. You can also use or modify a pre-existing file from a design library. If the 3D modeling software you use does not readily export to STL, you may be able to download a plug-in or extension that will do that for you. Or you can use another software program for that step.
Free 3D modeling software