Extreme obesity

I don't know,but I'm guessing swimming will help.No weight on anything, because of the water and will toughen up the ligaments,leg muscles etc

So as they've already advised there are 2 ways to work on this.
One is getting yourself a mobility aid like a wheelchair or scooter, which will deal with the issue most the time but doesn't help in those times you have to get up from it.
The other option is to work in some light exercise to strengthen your legs. Swimming is the best option, the water takes much of the strain off your joints. Laps are fine but if you wanted to try squats or lunges in the shallow end the water would definitely make those easier. If you think you can keep up a water aerobics class is generally designed with someone with joint issues in mind.
Regardless of how you choose to go, for the foot pain I'd advise visiting you local running shop and getting a good pair of shoes. You will feel out of place there but they'll have shoes meant to pad the foot under some serious pressure which it sounds like you need.
For the weak knees in the mean time consider a bariatric cane. I know nobody ever wants that option but it sounds like you need extra support on the left side and that will give it to you.

Let me tell you... going to a running store and having them fit my feet to the right shoes and recommend special insoles has made a HUGE HUGE difference in my mobility & ability to stand. (esp. the insoles) A decent running store won't shame you, will really know their stuff (they'll probably have you walk around barefoot, they may have a special machine that makes like a model of your feet), and while it might cost you $100-$150, this is an investment. (And if you wear out the shoes faster than you can plunk down another grand, you can always re-order the same shoes/insoles online at a fraction of the cost.) A friend of mine who's a runner has been taking me for years... at first I went from being able to stand for an hour or less to being able to stand 3h or more. (I'm bigger now and more out of shape and less mobile, but w/o these shoes & insoles, I don't think I could get around at all.) (Also, running socks are just the BEST. SUPER sweat wicking, they feel SO soft, and they hug your feet in all the right ways.)

Also, I know a lot of people don't wear shoes inside, but you need those insoles for support, so make sure you wear them inside too.

So, yes, I totally recommend going to a running store & having them fit you & find the right pair of shoes+insoles for you! (FWIW, in the Washington DC area, I recommend Fleet Feet Sports, esp. the Kentlands Gaithersburg location. Apparently Fleet Feet is a chain, but that's the location that has always given me excellent customer service.)