Tinkercad is part of Autodesk's 123D family of free apps.
Sketchfab publishing isn't natively integrated yet, but Tinkercad can export formats that we support: STL and OBJ.
We recommend OBJ + MTL.
Go to Export ("Download for 3D Printing") in the top right menu.
Choose OBJ and download the zip file.
You can then upload this straight to Sketchfab!
CAD software does not export textures accurately because they don’t use UV coordinates.