Its hard to add onto what erockomania, but I would suggest, if you can, start your diet today! Some people like to slowly move into a set diet, but that means you already lose like a week of being on a proper diet.
Are you doing any kind of training? Doing bjj is not to bad on the knees. Now its hard to say that for you, cuz I don't know how bad yours are, but mine are pretty shitty also. So when you start feeling pain, do stop. Try to do very light weights if you decide to work out your legs and strengthen those knees, its been helping me.
Also, start recording what your weight, take measurements of your body and if possible body fat %. This way you can look to see if your improving or not and change things accordingly.
A little advice, if your really training hard and loosing a lot of weight fast, don't get too excited, it might be water that your not recovering, so make sure like erockomania said, drink plenty of it.
Oh If I can add one more thing, try to get a few books on diets, workouts (arnolds diet section in his book is really good), theirs even a nutrition book for wrestlers. Reading these things really helps you understand what carbs, protiens, fats really are and you'll be amazed that you even have to mix a few foods in order to get the max protien from them. Also from what I understand carbs are best taken after a workout.
Lots to learn
Keep us updated on how your doing
