There a countless 3D files already created and ready to print.  Thingiverse is a great source for free files.

If you want to create something from scratch, there are many free CAD (Computer-assisted design) programs available.  If you are new to designing in 3D, we highly recommend Tinkercad.

Tinkercad is a free, online 3D modeling program.  There are tutorials available on the Tinkercad site to get you started.

Here are some excellent additional tutorials from PromoAmbitions.

Tinkercad Tutorial Part 1 - (Interface and Movement) - https://youtu.be/2JFxtUIOnEI

Tinkercad Tutorial Part 2 - (Shape Manipulation and Scaling) - https://youtu.be/Xy6EuqocJKI

Tinkercad Tutorial Part 3 - (Group, Hole, Align, and Flip Tool)  - https://youtu.be/hbQww7pWGt4

Tinkercad Tutorial Part 4 - (Workplane, Ruler, Blocks and Bricks) - https://youtu.be/aa2uboMStVQ

Tinkercad Tutorial Part 5 - (Circular Patterns and Slicing) - https://youtu.be/J_M8LGsElJs

Tinkercad Tutorial Part 6 - (Import Export and Collaborating) - https://youtu.be/1sVJZE2oVHs

Additional CAD programs