The instructions for cleaning a garage shelving unit will depend upon the material from which it's built. Metal and plastic shelving units are relatively easy to clean -- just use all-purpose cleaners or degreasers and scrub them the way you would anything else. Particle board and MDF shelves are more likely to stain permanently if they're subject to spills. For that reason, it's worth covering them with plastic sheets or contact paper than can be easily replaced when they get dirty.