The Scene Tab, located within the 3D Settings Menu, is represented by the gear icon. It includes options to straighten your model, select from renderer and shading options, restrict camera view within the 3D viewer, show your model's wireframe, keep vertex colors for certain file formats, and change the color or image of your background. Limits to options based on account status will be specified in the menu.
Some 3D setting options will only make sense to those with more experience with 3D models - that's ok! Play around with the settings to see how it changes the way your model looks. You can always learn more by visiting the resources listed in the Resources Tab of this guide.
Sketchfab's preset settings are most likely sufficient for your model. For this tab, the most important setting you might need to adjust is "straighten model," since your model may have been uploaded with an incorrect orientation.
Don't forget, you must always "save settings" when making 3D Setting changes or you will lose your work.
If your 3D model was loaded facing the wrong direction, you have two main ways to straighten it in relation to the gridded plane upon which it sits. In the example below, you can see that the model was uploaded sideways.
Under the "straighten model" header, you can select the arrows next to the "X", "Y", and "Z" boxes. This will rotate your model 90 degrees along each of these three axes. This tends to be a good way to start to get your model in an upward facing position.
If you need to rotate your model under 90 degrees along any axis, you can also enable the "show advanced rotation" box. When you select this option, you will see the plane your model sits on, as well as a sphere of red, green, and blue arcs that allow you to rotate your model as needed. Select and drag along the arcs to rotate your model along each axis; select the blue arrow and drag up or down to raise or lower the plane.
Compare the following images that show rotation only using the X/Y/Z rotation (top) and then after using the advanced rotation (bottom).
Note that if you just grab and rotate your model without going through one of these processes, you are rotating both the model and the plane upon which it sits. The orientation and plane affects how you and your viewers can rotate your model, so you want to ensure your model is properly straightened on the plane itself. In the image to the left, the model and plane have been rotated together without straightening the model using one of the options above. The plane appears behind the model, so our view is in fact looking down upon our model from above. Based on the restrictions to rotation, it is not possible to see the front of the model without straightening it.
When you're happy with your model's orientation and the camera view, select the "Save View" button at the top left of the viewer. What you see in the viewer will be saved as the first preview image of your model before and after it loads. You can change the preview image at any point in 3D Settings.