The SketchUp exporter lets you publish 3D models straight from SketchUp to Sketchfab.
You can install the SketchUp plugin directly from the Extension Warehouse. This is the best way to install the plug-in because it will automatically stay up-to-date.
If you prefer to install manually:
For detailed instructions, follow the official Sketchfab Uploader Manual, as well as this great tutorial from Alex Schreyer and this forum thread about model performance, to prepare and upload your SketchUp model.