To summarize -
1) clean. A hose or light touch with a pressure washer if its really dirty. We also talk about commercial cleaning solutions and a home brew solution with TSP and bleach. Bleach is only required if stained from algae or mold.
2) let dry
3) stain - Stains have different amounts of colorant or solids. The more color or solids the better the protection, generally speaking. A more natural stain will have less color or solids. Best compromise is maybe a semi-transparent.