Repainting your home's exterior is the mother of all home maintenance projects. Exterior painting can literally take half the weekends of your summer to properly prepare the surface and slap a coat of primer and two more of paint onto your house. Given that kind of time commitment, it behooves you to stay patient and do the job right. It won't just look better: a good exterior paint job will also wear better, lasting years longer than paint hastily applied.
Top professional painters will spend as much time preparing the surface as they do painting. You will want to go over your home's entire exterior, first with hand scrapers to remove any loose paint and wood, then with a block sander so that the new house paint will adhere to the existing paint.
The scraping process will undoubtedly leave obvious wounds behind on some of your shingles. If you just paint over these spots they will age quickly. Use a wood-epoxy putty to fill these areas in, and then sand it smooth after it dries.
At this point, a gremlin will begin whispering in your ear that you don't really need to prime. Don't listen to it. It is true that your house will look great with two coats of new house paint whether you prime or not. But your new paint is formulated to adhere to primer, not to old paint. If you forego primer you'll notice the paint chipping and peeling after only a few years. Similarly, applying one coat of paint instead of two will significantly shorten the life of the paint job.
Remember, you don't want to be doing this again any time soon.