3D objects come in many file formats but the printers use what is called an STL file. These can be created through apps online programs and stand alone programs below are a list of a few places but the easiest place to start is TINKERCAD online
Autocad
Mudbox
3Ds Max
Adobe Character Animate
Blender
Thinigiverse ( app or website for creation and also a repository for premade designs ( its huge!)
Makerbot the 3d printing company has an app
Octodroid (also made for the octoprint printers but files can be transfered elsewhere
Sketchup mobile (the website version is also great!)
edrawings used for mechanical drawings taken from pictures of objects.
Blockify ( paid app)
Tinkerplay ( uses a repositore of premade shapes to create objects)
Thingiverse is the largest repository of premade downloadable STL files you can almost find anything there but we encourage you to make your own that is half the fun!