Find designs online or create your own

You can find and download 3d designs online

https://www.thingiverse.com/ 

https://www.youmagine.com/

Find the design you like, download the files, extract the .zip file.

The .stl file is the file that you will use to print.

If you want to create your own designs or modify the existing ones you can do it with https://www.tinkercad.com/