My dad has has a stroke and shows signs of dementia, how can I get power of attorney over his affairs?

My father has had a stroke, and has been ill for 28yrs. His mental statis is very bad. How do i get power of attorney,for any of his affairs, such as his social security check? He dosen't care if he spends it or not. He will not listen to anyone, not us or even his doctors. Our situation with our father is very bad. Please help I don't even know where to start.