There are two ways to create your own 3D models and upload them to Sketchfab: modeling and scanning.
3D Modeling is a creative process where you can build a model (character, creature, vehicle, etc.) in three dimensions (x, y, z) as opposed to simply drawing or painting a picture in two dimensions. There are tons of different software options available to do this and they vary in terms of complexity, price, and system requirements. If you'd like to explore this route, you can check out 3D Modeling software options.
3D Scanning is the process of using multiple 2D images or photographs to create a 3D model. For example, Sketchfab users scan their dinners, cars, or even statues in museums. You can do this with a professional-level camera or even your smartphone! Just like 3D modeling, there are tons of 3D Scanning software options.
Once you make a model or two, you can upload them to Sketchfab and share them with the world.