Sketchfab publishing isn't natively integrated with Tinkercad yet , but Tinkercad can export formats that we support: STL and OBJ. We recommend OBJ + MTL.
To export an OBJ file from Tinkercad:
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!
Note: CAD software does not export textures accurately because they don't use UV coordinates.