How do I fix a leak in this roof?
It is really impossible to correctly answer this question without discovering the root of the problem. Some removal of shingles and or siding may be necessary to inspect the flashing etc. If you find the problem in the valley or around the sidewall, i'd recommend using some ice and water shield under any flashing that may have to be replaced. It can be difficult to find these leaks, so start high and work you way down.  Good luck.