If you're lucky, all your house may need before repainting is a good, healthy bath. Wash it down with a hose, and go over stubborn dirt with a scrub brush and warm, soapy water. Or wash it down with a power washer. If you're not so lucky, then you just have to face the fact that a time-consuming and dirty job lies ahead of you. Do the job well, and your paint job will not only look better, but it will last for five to eight years on average.
Once you have removed all the loose paint, you should apply an appropriate primer to some of the distressed areas, especially if your paint-removal system has exposed raw wood or bare metal. The kind of primer you use depends on the kind of paint you'll be using later. For latex paint, use latex primers; for solvent-thinned paints, use solvent-base primers; and for metals, use metal primers. Not only do these coatings provide extra protection against the elements, they also form a firm foundation for finishing paints. Also, priming is always required when you're working on new wood.