the most common cause of decomposing facia boards is not having drip molding...the second.....the facia boards were not primed.
Assuming you have gutters, the first thing you will need to do is remove them, then remove your old facia boards. Check your rafter tails (ends) and make sure they are in good shape. Then install your drip molding. This is a 1" X 3.5" angle iron shaped piece of aluminum, usually in 10 foot sections. Overlap sections by 2 to 4 inches and put silicone or roofing cement where the moldings overlap to make sure no water goes in between them. Make sure you put them under the roofing felt. Then prime and install your facia boards, set your nail heads and fill with an exterior filler, prime those small spots then paint facia with two coats of your finish color..then reinstall your gutters.