3D "printing" actually refers to the gradual build-up of heated plastic filament in very thin layers—usually just .2 millimeters thick. Filament comes on spools that feed into the 3D printer's extruder, which is similar to the tip of a glue gun. The extruder typically reaches temperatures of 400–550°F to melt the different types of plastic filament. The tiny layers it creates stick together to form a solid object.
If you want to give 3D printing a try, you can either start with your own design or use someone else's (as long as it is meant to be shared—Creative Commons). The resources linked below are free to use or offer free options. Create an account of your own to use them.