Walk around your home and look for areas where paint may be chipped, peeling or flaking. Also look for any signs of water damage, bubbling paint, or areas that have been faded by the sun. These are the areas that you need to touch up.

Angie's How To Touch Up Your Exterior Paint
Wet winters, heat-intensive summers and heavy storms are just a few natural elements that can wear down your house's exterior paint. Protect your siding, trimming and gutters by touching up the exterior paint of your house once a year.
3 pros!
Once you've inspected your exterior siding on foot, grab a ladder and examine the upper siding and trim. Look for the same red flags in step #1. Before you climb your ladder, it's a good idea to read about proper ladder safety.
To do this, grab a paint scraper and remove paint that isn't tightly adhered to the siding or trim. Don't be afraid of removing too much – if the paint is coming off easily with the scraper, it isn't protecting your house from water damage and other elements anymore.
When you're done with the scraper, use your wire brush to remove any remaining paint flakes. If your siding still has some dirt and grime on it, clean it with an old rag and warm water. You want the surface to be as clean as possible before you do any touch ups.
If you have the original paint you painted your house with, that's great! However, there's a chance your siding may have faded so the colors will no longer match. Scrape off a quarter sized chip of paint with your paint scraper and take it in to your local home improvement store to get it matched. You'll also need a good exterior primer. Before you begin painting, make sure your siding is thoroughly dry.
Read the instructions on the label before you begin and follow them exactly. Let the primer dry completely before you apply the touch-up paint.
Use the same paint applicator that you originally used to paint the exterior of your home. If you're not sure what tool was used, consult your local home improvement store when you're purchasing the paint and primer and see what applicator they recommend.
When applying your touch-up paint, make sure you spread a little bit of new paint around the area that is being re-touched so that it blends with the rest of the exterior (commonly called diffusing). This is usually best done with a brush.
If the color is too light, try applying a second or even third coat.