Suggestions for diet?

I have been on prednisone for 1 1/2 month due to a medical condition that I developed. That that I am ok and the dr has taken me off I have gained so much weight with it. I have ordered Pilates equipment and video and etc but as far as diet I don't know what to eat or what. I have never had a weight problem so this is all new to me. If anyone has any suggestions of a diet or food to stay from or whatever please let me know. I am wanting to lose about 30-40 lbs that I gained. I am very determined also.