You are wise to want to retain some muscle mass while you are losing weight. When I say "muscle mass," I don't mean bulk, but rather lean muscle, which is important for overall health and strength, as well as the definition that you are looking for.
A well-planned nutrition and exercise program will enable you to achieve your goals. From a nutrition perspective, you will want to eat a well-rounded diet which includes approximately 65 percent carbohydrate, 15 percent protein, and 20 percent fat. If you are trying to lose weight, you will want to make sure that you are taking in fewer calories than you are using every day so that you put your body into a caloric deficit so that you can lose weight.
The key question then, is "what should my caloric deficit (the difference between the energy you take in and the energy you use) be?" That depends on
While you may want to lose weight fast, know that if you cut calories down too severely you may drop some pounds quickly, but the likelihood of keeping that off is slim to none. You are better off cutting calories a bit more slowly, aiming for a weight loss of approximately one to two pounds per week, which has been proven time and again to be the best method of keeping weight off over the long term. In order to do this, a good goal is to put yourself in a caloric deficit of approximately 500 calories a day which you can do most effectively by eating less and exercising more.
Your exercise routine should consist of a combination of aerobic and resistance training. If you are new to exercise, you will want to start out gradually, but work up to a point where some of your workouts are somewhat strenuous to make sure you are achieving maximum gains. Also, it's always wise to check with a doctor before you start any exercise program to make sure you don't do anything to hurt yourself.