SketchUp is a 3D modeling software primarily for architects, designers, builders, makers and engineers. You can create buildings, furniture, interiors, landscapes, and more. Plus, you can upload to Sketchfab directly from the application.
Once they're ready, the best way to upload your 3D models from SketchUp Pro to Sketchfab is through the Sketchfab exporter.
Some SketchUp Pro models fail or perform poorly in the Sketchfab viewer due to the way SketchUp Components and Groups are handled. (The biggest culprits here are trees and plants.)
To work around this, this Sketchfab forum post about the SketchUp Bomb extension.
SketchUp offers a browser-based version of the software called SketchUp Free.
If you made your model in SketchUp Free, you can only export the model as an STL file. Once you do this, you can upload to Sketchfab by clicking the Upload button in the top-right corner on any page.