how can I obtain authority over my elderly father's matters?

If you have brothers and sisters, it would be best to have a family meeting to discuss your father's ability to handle his matters. Together decide what would be best.

If your father is willing, he can give you POA (power of attorney) to take care of his affairs should he become incapacitated. You should also have a medical power of attorney to cover medical wishes. I have both over both my mother and my father to make sure that if anything happens I can take care of them and not have any iternference from the state.

Any good lawyer should be able to walk you through this with your father or parents. It is an easy thing to take care of, but really necessary as they get older and start to decline in mental issues...