In reply to rich in wales: Don't over do it at first. Your training must be progressive at 20 stone it might be better to learn to walk before you learn to run. Others very right about diet - cut out booze (or cut down), avoid manmade sugery things like chocolate/cake etc switch to natural type stuff like fruit dates nuts etc. Don't starve yourself but take it steady.
Exercise I would suggest finding walking routes you like but find testing distance wise and walk at least every weekend. Altough I hate them walking poles will be very useful to take the strain from your knees and help you maintain posture. Shorter walks or Jogs if you can during the week. Try basic yoga stretches (nowt gay about yoga these days) will help your core fitness. As the walking gets easier - carry more weight or walk further or walk up steeper stuff.
If you want to try running/jogging get a cheap heart rate moniter and it will prevent overtraining and pointless strain.
Take it easy. If you overdo it you will get injured/get bored and stop.
Ultimately if you intake more clories than you burn you will get fatter, I find the best way to think of beer is as a mars bar in a glass. Love mars bars and beer so do quite a lot of walking!
Good luck.