I must have a small roof leak in my 1.5 story, 2 dormer, roof.

There is a small darkening spot, about 6" in diameter, on our 1st
level bathroom. It's about right at the point where the 1.5 story
dormer meets the roofline. From that bedroom I looked out onto the
roof and see the shingles as they come up to the roof nailed at the
very top where the roof meets the dormer. The top row of shingles are
nailed and the nails are exposed. One nail in particular seems to
have broken through the shingle. I'm thinking I need to start there
and somehow seal that nail. How should I do that?

What else do I need to look for? How should the roof shingles as
they meet the dormer walls be installed? It just seems like that is a
leak waiting to happen with a wall running down to a roof where it
cannot be properly capped. Is there some sort of flashing there?