I am not saying to turf your cat outside to get exercise, far from it, but you can create a cat gym at home, to get your cat moving and lose some of that blubber.
It is not healthy for a cat to carry around extra weight, so one of the first tips is to be more careful with their food supply. Many people leave dry cat food out for the cat to graze during the day. Dry food has calories and fat too, and should be limited if you have a fat cat.
Take your cat to the veterinarian for a checkup first. Find out how much your cat weighs now, and how much he/she should weigh.
Read the package directions for how much your cat should be eating per day. It should list weights and amounts of food. You should look at the weight your cat is suppose to be now, for the right amounts not the weight he/she is now.
Take that amount and divide in half. Feed your cat 1/2 in the morning and the other half in the evening. Once your cat has finished the serving that is it until next feeding. It may take some getting used to, but now you are going to distract your cat from food, with some fun and exercise.
Make some cat toys that include catnip. Purchase catnip (most pet stores) and cut two squares of bright colored cotton, about 2 inches square. Sew them together leaving an opening to stuff with cat nip. Then sew it closed. You can make a few of these. Then let your pet smell it, and then throw it. She will run after it and possibly bring it back or go on to make her own game.
You can get catnip filled toys at the store, but the homemade ones seem to be a bit stronger in scent. You cat will go crazy for it. Throw the squares around the room, and watch her/him run and play.
Make sure you have plenty of fresh water out for your cat, as this is thirsty work! This also gives her something to play with when you are not home, and distracts her from food.
She/he will get lots of exercise picking and climbing this tree.
If all else fails, and you just can't get your fat cat interested in anything, you can do what I did, and get a second cat! They will keep each other company, chase each other around the house, especially when you are not there. Make sure and put away your breakables! Your fat cat is about to get fit!