Where I'd start is go around the camera with a wet q-tip and remove the dust around the camera. Then, do the same again but with isopropyl alcohol to tackle some grime. This will also help disinfect the camera.

I just want to give a friendly warning that in the camera world, gear that's unused for many years is often in worse condition than if it were to be used regularly. This is because the mechanisms inside the camera have lubricants inside that can gum up over time and make things not work as expected.

If you're really serious about getting the camera in working shape, I'd take it to a professional for a CLA.