All siding gets dirty, but cedar siding, often used in humid parts of the country, has more of a tendency than other siding to support the growth of mould and mildew. Unless it is really bad, cleaning is basically done in the same manner as for aluminium or vinyl siding, with much the same products. See "Cleaning Siding" for the cleaning formula.
If the wood is badly infested, or unsightly and old, you can use deck "restoring" products that do more than just bleach off the surface. These actually dissolve a bit of the old wood, and take the mildew with them.