Preparing my roof for new shingles?

I have one layer of shingles and need to put on a new layer as it is starting to leak in places. It's the original roof and is about 50 years old. The problem is there is a very large pine that overhangs a portion of the roof. While I have trimmed back the offending branches, the damage is done. A little ecology has sprouted up, and there is quite a bit of moss and lichen on this section of the roof. My question is, what do I need to do to prepare the roof for a second layer of shingles? Do I need to remove/kill the growth, and if so, how? I would like to avoid having to remove all the shingles.