Step flashing for a leak free roof !
The biggest problem I find when I get called out for a leaking roof is where the roof meets a wall. A dormer or a chimney needs a piece of step flashing for every shingle along the edge. A step flashing needs to be at least an 8" x 8" piece of metal bent at a right angle in the center. Lay it on top of your shingle from the tar line up. Lay a shingle on top of that and repeat. This turns the water back out on top of the lower shingle instead of letting the water pass under the shingles and soak the wood under them. Too many times I have seen a single piece of aluminum run up the edge of a wall and the shingles were wet underneath, and the wood was rotten. Worse, no metal, no flashing, just shingles butted up to the wall and maybe tarred. Tar is not a structual patch. It cracks and peels.
Also remember that step flashing needs a cap. Vinyl siding on a wall or grooves cut horizontally in a brick wall and a cap bent to fit the groove, will cap the top of the flashing. A bent cap can be made of aluminum trim stock. It comes in many colors and in widths from 8" to 24" wide.
Even if you call a pro, ask him about step flashing and specify it on your house.
Tom in KY, stay dry.