Where do I getthe key to the door in the sewer with rats?
There are ashes near the white tree the giant throws spears at. Explore that area. If you
talked to the giant and got his branch, or chose it as your gift, he won't hurt you.
is there to forgive you for going down the dark path so you can still get the good ending instead of the alternate one.
To stop hollowing
you need to use a purging stone to reverse it and lower your curse meter to 0 (it's the counter underneath your level on the player stat screen.
For every dark sigil you have in there, you gain 5 curse points per death. 15+ curse points you go hollow. Yoel gives you one dark sigil for every time you took a free level from him.
you need the fire keeper to remove the dark sigils. You can pay her souls to do this. She gains the ability to do this after you hand her the soul of a fire keeper, which is found in the tower at firelink shrine (unlocked with the 20,000 soul key)
Purging stones
Yuria of Londor
Yoel died before you took 5 dark sigils
Profane Capital