I agree-baby steps to a healthier lifestyle. If you currently don't exercise or eat well at all, then don't try to do 3 hours of exercise and live on fish and salad right away....you won't stick with it.
A good starting point, since you are inactive, is 15-20 minutes of activity a day. Make this a "habit". Do it for a few weeks...and when you do it regularly, and stick with it, then move it up a bit. It can be walking around the neighborhood or on a treadmill, doing a dvd-or you can even sign up for a fun class twice a week or something. It doesn't have to be aerobics if you hate aerobics...or jogging if you hate jogging. Take a dance class, a soothing yoga class, or anything. Walk around the local mall and window shop/people watch while you do it. If you choose something you like, you will stick with it.
As far as eating better-baby steps as well. If you don't drink water...don't try to down 8 glasses a day the first week. Start with 3 or 4. Once that is a habit, step it up a glass...then again in a couple weeks. Change your habits SLOWLY...once they become your "lifestyle" then move on to other changes.
I eat chocolate. I am addicted. LOL I just don't eat a king size candy bar any longer. Portion control. I might have a mug of diet cocoa instead...saving almost 200 calories. Small changes, without deprivation.
If you have a fast food addiction-the first change, rather than cutting it out completely-is portion control. Instead of the large size value meal...get the kids meal with a Diet soda instead. Same thing, smaller size...so less calories. Portion control is a BIG thing.
Over time, start eating more veggies or fruit for snacks and side dishes-and less of the mashed potatoes/fries/breads.
No one is doomed to be fat. It is about changing your lifestyle, rather than "going on a diet". A diet has a beginning, and an end-and when you end it, you gain weight back.
Lifestyle changes are the key.