There will be posts to follow and I'm sure you will find help here. Your very 1st priority before anything else is to locate and STOP the leak if you havent already done so. You seem to know that it is your drain hose (Can you see it?)
If you know its the drain hose then take it off and carry it with you to a home improvent outlet or hardware store and match it up. They are pretty cheap. If you already stoppped the leak then you will need to remove the shingles/siding (Please let us know what type siding it is). I assume wood since its rotting.
After the shinles/siding are off then you'll know the extent of the problem. If your handy you can be walked through it. Do you have tools? or access to some? Type of shingles? How old is the house? Which floor is it on (1st or 2nd)? How big is the area rotting?
Get back with the answers and we can go from there.