where and how do I begin ?????

I am 74 years old.
Have knee pain saw an orthpedic Dr. was told of knee problems and had Arthrosocpic procedure a year ago… plus I was told I have arthritis. I was then referred to an arthritis dr. And now I have take some pain medication that helps my knee pain. I sat so much over the past few months that when I do get up and walk, the back of mine is so weak.
I tire very easily. I want to know what I can do for that back pain.
I am unable to get down on the floor yet.
What can I do to strengthen up the back?