Welcome to the forum.
You don't mention your height - but unless you are very tall it sounds like you are embarking on a major weight loss project. My max BMI was 52 (max weight 294 at 5ft3) and my husband's max BMI was turned 62 (max weight 409 pounds at 5ft8) so I probably have a fair idea of where you are coming from...
The best thing that I can do is tell you about what worked for me... I went out walking every day... it turned out that walking was an absolutely wonderful form of exercise - even slow walking (and when I was big I didnt do anything fast)... On day 1 I went out and walked near where I lived - a route that was about 0.6 of a mile... I went slowly - and paused when I wanted (and sat on people's garden walls)... I did it - and felt proud of myself for doing it...
The next day I went out and did it all again... I added distance as soon as I could... I walked a bit further... to the nearest shop... then increased it as far as the post office... I just walked every day and made it further as soon as I could...
Amazingly soon - I was actually able to walk some pretty impressive distances... I got a pedometer and allowed it to challenge me into walking more steps than the previous day...
At the same time I stopped eating chocolate and things like that... takeaway food got axed... bit by bit I improved things - in lots of seemingly small changes...
I tried to eat healthier...
I started weighing myself every day and made a note of what I had eaten and my weight so I saw what I had on days where my weight went down and also what I had when my weight went up...
Things that gave me a weight loss became things that I would want to have more often...
If I saw a consistent repetition of weight gain - I axed things from my food plan...
You are looking at a long term project so you do not want to think of it as a diet - but more a case of adopting a new healthy lifestyle... A new way to live for the rest of your life.
The following link may help you to settle in.
http://weight-loss.fitness.com/threads/57955-My-advice-to-newcomers
Good luck with your project