Flashing is a good way of preventing water from entering a dwelling at the point where the roof meets the walls. To begin the installation, fold a five-by-seven piece of aluminum flashing lengthwise. Start at the bottom and work up, nailing high on the side wall and interweaving the flashing with the roof shingles. Overlap the pieces of flashing and repeat this process all the way up.
For more on roofing, consider:
Bob Vila's Guide to Roofing
Leaky Roof? Some Repair Tips
Roofing Roundup: 7 of Today's Most Popular Choices