Onshape is a cloud-based, 3D CAD document system.
Onshape can export models in several formats supported by Sketchfab, including glTF, OBJ, STL, Collada, and IGES. We recommend glTF.
Right click your Part/Assembly and click Export... from the context menu. Or, click Export tab... from the Part tab menu.
Choose GLTF as the Format and any other options as needed (details here).
You can upload this glTF file directly to Sketchfab.
Then, you can adjust the camera, materials, etc. as needed.