By literally eating less junk food.
You can learn to substitute the junk meals with healthy meals. Learn about healthy nutrition and some healthy delicious food recipes.
Vary the food you eat, so you don't get bored and crave the junk food again.
It might be difficult at first to change a habit, but on the long run the body will stop craving fat and sugar and will find satisfaction in eating healthy nutritious food.
You can allow yourself to eat junk food once in a while for a change, but try to keep it in control.