The biggest problem that faces most people with painting houses is peeling, moisture is the culprit. Whether it's rain or just morning dew it'll penetrate the paint causing it to first crack which in turn results in peeling. Paint peeling is often found at the boards edges.
Cracking is another problem with older homes, that had lots of paint jobs already. Cracking can not be really fixed effectively. In places where you have serious cracking it is best to strip that area clean to the raw wood, sand and clean the raw wood off and then prime the wood.
Step one-scrape down the area that you want to paint. I suggest not to scrape only in one direction, but in a variety of different directions because if you scrape the one direction you might not be lifting the edge up that is still loose. So if you go in the opposite direction this should remove the loose edge. Visually inspect the scraped area for any more loose or loosing edges.
Step two-once you're confident that the area has been completely scraped of loose paint you should sand the edges between the sanded paint and raw wood to feather out the remaining edge. Do not over sand the bare wood because this will create an indent.
Step three-choose a good high-quality exterior primer. If your house paint colors are a light color then you can stick with a white primer but if you have a medium to dark exterior color I would have the primer tinted to match as closely as possible the finish coat.
Step four-applying the primer can be done with either a brush or a roller. If the whole job has a very nice finish on it you might want to use a low pile mini roller to eliminate brush marks. If you do end up brushing the bare spots, do not put it on so thick that it leaves heavy brush marks.
Step five-now that you have let the primer dry for the day you can put on the finish coat, again with the same method as before. To re-coat wait a few hours and apply again.
These are the very basic steps for a simple scrape and paint job. Later I will cover things like; house paint colors which will discuss ways of deciding on choosing house paint colors and then I will discuss a topic on house painting interior where I will cover everything from choosing colors and types of paints. you should consider painting your house.