To send a "print job" to the 3D printer, you have to have a ".stl" file. Many of these have already been created and are free to download.
These are sources of ready-made designs. You might find exactly what you need, or just use these as a way to get your feet wet in the world of 3D printing.
You may be required to create an account. Many files are free to use, but some may charge to download.