You should certainly protect your deck from unnecessary moisture intrusion and the harmful effects of the sun’s ultra-violet rays. Use of a penetrating stain, containing water repellant ingredients in most cases will protect your deck best. You may also consider using a clear water repellant, but this will need more frequent maintenance. The pigment in the stain will absorb the ultra-violet rays from the sun and protect the wood fibers from degradation. In most cases a stain will need maintenance every 2-4 years while the clear sealers require maintenance every 1-2 years.
Prior to finishing the deck it is important to wash the deck thoroughly to remove dead wood fibers, dirt, and any mildew spores. It is important to note that inactive mildew spores can be present and not be visible, but still should be treated.
One final note, if you plan on cleaning your deck with a pressure washer take caution not to use excessive pressure or spray at an improper angle. Improper use can severely damage your deck and cost you much more than you saved by undertaking the project yourself. Take the worry out of this type of project and hire the professional contractor.