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.