At the very least, you should replace the foam seals. By now, those are either crumbling, goo or both. Jon Goodman sells kits on eBay under the name "interslice." Good kits, especially if you only have to re-foam one camera.
Otherwise, clean the mirror, and if you can clean the focusing screen (very carefully). I don't know if those are user replaceable, but if they are, remove it and clean it under running water with a bit of dishwashing soap using your fingers. Then let it air dry or dry it using compressed air. Be very careful, because these screens are plastic and easily damaged.
Also, clean the underside of the prism before replacing the screen. That is, if you can remove the focusing screen.
Ideally, you would want to pull the top deck and clean the eyepiece and prism, but that might not be easy to do for a first-timer.
Also, clean all of the outer lens surfaces.
Clean the body of the camera. I usually use a washcloth and dishwashing soap. If it's heavily soiled, you can use a soft-bristled toothbrush. Then wipe with a clean, damp cloth and dry with a towel.