Substitute. Most junk foods have really good substitutes. Ex: Fat free greek yogurt is a great sub for sour cream. Baked french fries are really good instead of fried. Dip cucumber slices in yogurt instead of chips. Flavor the dip anyway you want. Frozen banana chunks instead of ice cream. A little time and attention and you don't really have to give up anything. The internet has great substitute suggestions.
Junk food can become addictive and isn't always easy to stay away from. I suggest you stop very slowly. If you suddenly withdraw from junk food, you'll eventually have a day where you give up and eat as much junk food as possible. An easy way to get off slowly is to start out with sugar substitutes (such as honey) as a replacement for REAL sugar. Then, just consume less and less as you go on.
Eat more sweet fruits when you have a craving for junk food and add more protein and fiber in your diet. A little bit of willpower will help. Take it slow. Gradually reduce junk food. Don't go "Cold Turkey".