Out of shape, short of breath - how do I start getting fit?

I am a 65-year-old woman, quite overweight, with a number of issues that have affected my mobility for the past 6 years. I have arthritis in my hips, feet, and both knees -- had my left knee replaced 2 1/2 years ago; my right knee is now painful, but I want to avoid surgery for as long as I can. I also had severe sciatica that prevented me from walking for most of 2010. By the time I finally had surgery, there was nerve damage down my left leg, and I still have pins and needles and weakness in that leg and foot. So walking is still a challenge, but I'm more mobile than I was. Being unable to walk without pain for all those years has made me quite sedentary, and now I get short of breath after very little exertion. I find walking uphill, even a slight incline, especially difficult - and I can't walk uphill and talk at the same time. We live at the top of a (fairly gentle) hill, and although the centre of our village is less than a half-mile from our house, I now find myself driving there & back rather than struggle up the hill to get home - which certainly isn't helping my fitness. I know I need to build up fitness gradually, but I'm not sure how to do it safely - or how gradually to do it. Help? Thanks!