Where to start exercise for a really overweight person?

Walking is great at first, gets you mentally ready to do something on consistent basis, it can actually be a strength training workout at first and could gain muscle in legs doing it.

But probably good idea to do some upper body resistance training, because a diet, especially as extreme as you are hitting, will cause muscle mass loss.
That's roughly 50% deficit even when sedentary. More if you only eat that and exercise much.

For upper body, pick a template and routine here, then pick exercises you have ability to do with what you have.
http://www.exrx.net/WeightTraining/Instructions.html

So far, you have started in the right place. Walking is a great exercise for everyone, but is especially good to start out because you can easily adjust and increase, depending on how you feel.

Do you like water? Is there a pool close to you? A water aerobics class is really good because it encompasses cardio and full body strength workouts without as much wear and tear on the joints. Lap swimming, aqua jogging, and even treading water for a long period of time are great for low to no impact exercises. Other types of strength and lifting can be really good too, but I'll let others chime in on ways to get started with those because I don't lift.

Its great that you are walking. In terms of strength training, you can do some pilates and use a resistance band with light strength then gradually work your way up. You can also do beginner workout videos at home and look at reviews over at Amazon and Video Fitness Forum, with of course, consulting your doctor. I hope this helps.

But yeah to keep things interesting, do a mixture of different things, so you can stay motivated, learn different cross training methods, and not get bored

Best wishes~

Walking is good. You might want to consider also getting a Fitbit or an activity tracker like it. They can go a long way in helping to motivate you to move throughout the day. Before I got a Fitbit I might have walked a mile or so on a typical day. The Fitbit showed me how lazy I was, and that one mile quickly turned into 5 to 10 miles.

Strength training would be a good idea too. As noted above, it'll help you maintain muscle mass while you're losing fat. And at 440 lbs, you've got to have some serious muscle underneath the fat, otherwise you wouldn't be able to get yourself out of bed or out of a chair, much less walk 1.2 miles. Save as much of that strength and muscle mass as you can while you whittle yourself down to your ideal weight and proportions.

Walking is a good start. Hop on a bike, maybe try out of of the elliptical. But, I do echo the sentiment of getting a weight training program going alongside your cardio. You're going to lose both fat, and muscle mass when losing weight, so you want to minimize the impact.

I was 5'4'' 292, I started out walking and then I got a recumbent bike and occasionally used a elliptical. When I got bored, I tried to follow along with videos on youtube of Jillian Michaels and the like. I picked up the Just Dance 2014/2015 game- super fun. And I started playing volleyball for fun and on a league. Started jogging with C25k app. I just liked to keep things interesting as I do get bored.

Don't try to walk fast. There is another thread about heel spurs, lol. You don't want one and walking fast when you're overweight and not used to it is a great way to get that problem!!

After the doctor clears you for exercise, start immediately. Walking is great. Swimming is fantastic! Nothing burns more calories, it's easy on the joints, you don't feel hot and sweaty and the water is one place where the fat will not hold you back! Start swimming and watch the pounds melt away!

There are a lot of people who were heavily overweight who only used the fitbit or pedometer to reach their goal weight. Its not strenuous, its easy and its very motivating seeing those steps go up. I cannot advise you on the strength training, but thats what the other folks are here for. The community forums are great!

As you progress, remember to change your diet accordingly. Anyway, good luck!

start walking daily

Walking, definitely. It's the easiest way to start. If you need goals to work towards, you can set mileage goals or even mile pace goals for walking, that way you can measure your progress. As you build your endurance, you might find that you want to try running and C25k programs are excellent for starting out trying something new. Good Luck!

Very good start! I will tell you for less damage and a lot less pain before, during and after... use a pool, join the YMCA in your area and workout in the water, you can jog, do jumping jacks, lift (they make weights for water use) if you can't really do the Y try a local college and see if they have a pool and what it will take to use it.... even some motels will allow you to use their indoor pools for a small fee...

Shep you are well on your way. Continue walking. Get into strength training. Plenty of body weight or free weight programs to help you out.

Personally, I started by just by focusing on my eating and getting that "under control". About 3 months in, I added walking3 time a week for about an hour (over my lunch time). I then increased it to 5 times a week.

Once you lose some weight and get more energy, start finding what you like to do. Check out Exercise classes, video series, even find a buddy who likes to work out/exercise and start exercising that way.

When you get closer to your weight, that is the time to start worrying about maintaining/increasing lean body mass and starting to think about weight lifting. Being a bigger guy you currently have a LOT of lean body mass and can afford to lose some. Trust me, as you get closer to 280, you will want to start thinking about it, for now, just focus on building life long habits.