Best beginner exercises

I started out by riding my bike for 20 minutes three times a week because it was easier on my knees even though it felt uncomfortable to be almost 300 pounds on a small bike seat. I slowly added in about 5-10 minutes on the elliptical because it was low impact and I get a better calorie burn than the bike. As I lost weight and my fitness improved, I have added a 4th day of exercise and my time has increased to at least 45 minutes. You may try that or swimming to have less knee pain while you eat at a calorie deficit to lose weight. Good luck!

My suggestion would be a good progressive bodyweight strength program, the one I do is at startbodyweight.com, and think it's among the best out there. You can lift weights alternately, but would do some form of strength training so any weight loss will be mainly from fat, and also you may even add a little muscle in places.

With that, I would walk, which is a nice gentle form of exercise, and personally very pleasant for me.

Between the two, your general conditioning should improve quickly enough without pushing things too hard seeing as you're coming from a level of inactivity. Once you feel like doing more then, you will know, and will probably have a good idea what you would like to kick on with next.

Squats will help your knees by the way, and a bodyweight program like the one above will generally help improve and strengthen your joints, connective tissues, balance, co-ordination, and a slew of other things. Mostly though, start at a manageable and sustainable level, make it a habit, and you won't look back I suspect

EDIT: re walking, I would say do what you can, but if it's too much, I would look at an indoor bike, or swimming as a low-impact exercise to start off with.

EDIT 2: Ooops, should have read your entire post before jumping in with a reply. Just want to add there are some indoor 'bikes' that are just the lower half, ie the pedals part, and you can use them by placing them in front of a chair you are sitting in. That may help if other indoor bike options are out, plus such indoor 'bikes' tend to be quite inexpensive compared to regular indoor bikes

I don't know what your particular mobility is, but let's say for someone who has been off their game for awhile, I might suggest the following:
Keep moving. Fidget. Get a couple Dumbbells. Ride that bike and give yourself challenges. Practice balancing poses or calf-raises when waiting for the microwave or while filling up your water bottle. Do arm circles during commercials. Just keep moving. That's going to burn calories.

And follow a recommended diet from a doctor/dietician. Most people will be able to move more once they start dropping pounds.

I know that taking control gets me on course to my ideal self.

Don't rely on anyone to get you to your goal.