In order to print, you need GCode. For 3D printing you can make models manually with GCode but it is far easier to design a CAD file first, and have your slicer convert it for you.
Finding a free model is easy! Websites like www.thingiverse.com are giant repositories for free files. Some websites require creating accounts or paying for printing licenses. For the most part however, you can find whatever you need by looking around.
Creating a file is a bit more advanced. Beginners can work off websites like www.tinkercad.com, and begin to learn good design principles from there. For more robust and complicated designs, Maya, Blender, Zbrush and other programs along those lines will suffice. In order to make the process as seamless as possible, the program should be able to export to STL, OBJ or PLY. If it can only export in drawings (dwg) or another 2D format, meshes can be disjointed and faces can be lost in the conversion process.