You can google that question and get many marvelous ideas, but I thought I'd share what works in our family.
1. Stop buying processed snack foods. I cringe when I think of how many boxes of crackers, granola bars, cereal bars, fruit snacks, puddings, and cookies we used to buy each month. I remember spending a pretty good portion of our grocery budget on snacks and wondering why we didn't have enough money for groceries.
2. Start slowly. First I quit buying snack crackers (we still buy soda crackers, because 2 of our family will only eat soup with crackers), then granola bars, then puddings, etc. I replaced the sweet snacks with my own baking- banana bread or muffins, zucchini pineapple loaf, etc. When they wanted a sweet treat, at least it was made with real food with my own hands in my own kitchen.
3. Replace lost foods with whole fresh foods. I started buying more fruit and vegetables, including some great "treat" fruits that are only available seasonally. Because I started this process in the summer I was able to fill their bellies with fresh berries, watermelon, etc, and gradually change taste buds around without them knowing it. If you plan the timing appropriately you'll have more success- if I had started in the middle of the school year it would have been much harder.
4. Keep fresh fruits and vegetables available and ready-to-eat. When someone in the house says to me "I'm hungry, what can I eat", the first thing I suggest is fruit or veges. And I'm not generic about it either. I will suggest specifically what they can eat- baby carrots, sliced cucumbers, orange, apple, strawberries, etc.
5. Identify why you're personally not eating more fruits and veges. We lead by example. If we say "eat an apple" while we're skarfing down a bag of chips, they're not going to be sold on eating better. My realization came about when I was deciding what to do with a beautiful crystal bowl that I wasn't using. I realized that I love fresh fruit- but I hate it cold. For me to love it, room temperature is the way to go. So now we always have a well-stocked bowl on our table. I'm refilling it like crazy. But that's ok!
6. Teach your family to cook. It's such a useful life-skill, and watching real food become amazing meals can change a family for the better. Plus, you can teach about a balanced meal by asking questions as you cook- here's the carb, here's the protein, what are we going for eat to fill the fruit and vegetable portion? Get your family in the habit of thinking about balanced meals.
In summary, if you fill your tastebuds with healthy, real food, your taste for high-calorie/high sodium processed/fast foods will become a thing of the past. It does take time, and we still eat out from time to time, but given the choice- if we're home, I'm going to cook. I like real food so much more than fast food.