New here, lots to lose! Need advice on exercise please!

Hi, I have been overweight/obese for awhile. I had some health problems (not weight related) that came up 2 years ago that caused me to gain a lot of weight, but that's just an excuse, I was well over 300lbs before any of that. In April of this year I weighed in at 372. I'm tall so I always try to convince myself that i carry it well so it doesn't sink in just how HUGE I am In May I had some more health issues (same issues, not weight related) and after having surgery on my legs they both swelled up so badly to the point I gained over 40 pounds in one week just from the swelling. I now weigh in at 414 (( I have a very hard time walking or moving. I was active and had no mobility problems before. I could walk a couple miles and go up stairs without getting out of breath. Now I am using a walker and can only walk short distances. How can I exercise and burn calories like this? I know this is a very long journey I will have, my goal weight is 150 but I just want to get under 200. I am hoping finding this board will help, I found this board through one of my Facebook friends posting a story about a guy on here who lost over 300 pounds in 3 years. I am so amazed and want to lose close to that myself, probably. Anyway...just want to introduce myself and put this out there...and also ask about how I can exercise when I have trouble even walking??? Thanks!