Secondly, Yoga, like other eastern philosophies, looks upon the abdomen as a center of energy - prana. Yoga will help strengthen your core from inside, including your organs, and do much more for you than any abdominal exercises.
There are plenty of abdominal yoga exercises which will help you get slimmer. To learn these, it is recommended that you find a good yoga teacher, or guru, who will guide you along your path. Yoga needs supervision, and should not be done by yourself until you are proficient, but we can tell you about some of the asanas and kriyas that you can ask your teacher about.
The simple Surya Namaskar or Sun Salutation will help you tone up your muscles and reduce belly fat. You can do these first thing in the morning, as many times as you can. Other asanas (or poses) that will help are Paschimottasana or seated bends, Urdhva Dhanurasna or Bow Pose and Sarvanganasana or Bridge pose. There are many other asanas that will help your abdomen, and different teachers teach it differently. Unlike abdominal muscle exercises like crunches, these abs yoga exercises (asanas) strengthen the whole torso, including the back and side, meaning that you will not suffer from the imbalance that an obsession with crunches brings.
The ancient yogis understood that breath is life, and that it should come from the bottom of the abdomen. Therefore, yoga also has a number of breathing techniques. Learning these will help make you stronger, livelier and healthier, and a slimmer stomach will be the most superficial of your achievements. Ask your teacher to teach you Anulom-Vilom pranayama, or cleansing kriyas that will help you get rid of toxins in your abdomen.
Even a simple yogic meditative pose like Savasana will help. When done properly, it will help you get in touch with your centre, and feel, first hand what is wrong, rather than relying on someone else to tell you.
If you want, you can supplement your yoga with abdominal muscle exercises and abdominal stretches, but remember that you will also need to strengthen your back, or it will be detrimental for you.