I used to do sports too, and I was prone to spraining my ankle. One thing I did to protect my ankles during my workouts was by using an ankle brace. You could try the same thing with your knee until you feel comfortable enough without a brace. You can buy a knee brace at
Wal-Mart or any convenience store and they're not too expensive.
In regards to eating habits, start small! For example make a gradual switch from 2% milk to skim milk or change from white bread to whole wheat/whole grain bread. Of course setting a low-calorie diet would be a great idea too! By making an effort to eat on a consistent basis, it will be easier to eat healthier. For example, for my diet I eat 400 calories every 4 hours.