First things first.
After your treatment, you should start by discarding any residual medication. Follow this up by nebulising clean water through the unit for a minute or two to clear the mesh holes.
What should you wash?
The important parts to keep clean are your medication container, mesh cap, mask adapter, mouthpiece and, if you're using one, your mask. These can be washed in warm, soapy water to clean. Rinse them off to get rid of the soap residue and leave to air dry on a lint free cloth.
As for the main unit, wipe it clean with a moist, not wet, cloth.
Never touch the mesh directly with your fingers or any implement such as a cotton swab.