cycling is a great way to get fit as it can be good fun, I'm out on the bike pretty much everyday.
I would recommend that you also do another activity so as to exercise other muscle groups. Cycling is non-impact so perhaps alternating between some easy running/jogging and cycling would help see better benefits than cycling alone. If going running, get a good pair of quality trainers (FYI www.amphibianking.ie
is where I picked up a pair)
As others may have said, take it easy at first. Build it up so that you don't get dis-heartened about not being able to go long distances and also so that you don't injure yourself by overdoing it.
Remember to do some stretching after you have finished your exercises. Some folk recommend that you don't stretch before you are warmed up (do a light jog first or whatever). However, you should stretch properly, so get someone knowledgeable to show you or pick up a book that explains the fundamentals.
Finally, food, as some one already mentioned, cut out the rubbish. It makes a huge difference. However, make sure that you actually fuel your body. When doing exercise you are going to need to eat the right stuff. I eat like a horse, but I only weigh 11 stone (6' 3"). I try to keep all junk food to a minimum and eat plenty of fruit and veg and wholewheat (rice, bread, pasta) and fish
that's all I can think of right now, hopefully it's all good advice. I'm far from an expert in the areas of sports nutrition/training so everything I've said/know are bits of information that I've picked up from others or from books/t'internet