Even though it is normal for a kittens' weight to double or even triple within the first few weeks of life and continue to increase until your kitten reaches maturity, this needs to change after she reaches maturity. Obesity is all too common in household cats, especially spayed or neutered indoor cats. This is not a reason to refrain from spaying or neutering your cat, but instead a reason to be attentive to your cats' nutrient and calories needs.
Water should be kept fresh and available at all times for your cat. If your cat drinks out of the toilet, it more than likely means you need to clean her bowl and replace it with fresh water more frequently, ideally once or twice daily. Stainless steel bowls are best, since they are smooth - plastic may feel smooth but it collects more bacteria than stainless steel.
Now, about the food, this is the hard part. To start with, you will want to choose a high quality food. Dry food helps keep tartar off his teeth, but it is your -and his- choice. Stainless steel bowls are best for food, and her food bowl should be washed after each feeding. Generally veterinarians recommend feeding adult cats twice a day, but you should always check with your vet. The amount of food your cat needs depends on which food you feed her, the amount of activity each day and her weight. Your vet needs to be the one to determine a healthy amount of food for your cat. This will change, gradually, with age, weight, food changes, and health conditions.
When you rub your cats' sides (rib cage) you should be able to feel the ribs by pressing in gently with your finger tips. You should see a noticeable abdominal tuck as she walks. If your cat is an outdoor or outdoor/indoor cat keep in mind she may be using her charm to convince your neighbors to feed her a few scraps or stealing food out of other pets' food bowls.