A few suggestions that have helped me....
1. Don't try to go cold turkey. People are different, but for me personally it was a heck of a lot easier to know that I could still have those things, "just not right now." I used to love chocolate milkshakes. I could still have them... I just needed to wait until the weekend when I relaxed a little bit. Eventually, I gave them up altogether, but in the meantime it was much less traumatic and reduced the cravings massively to reassure myself that I was not going to be permanently deprived.
2. Eat more fat and protein. These are good for you, and will power your body efficiently. They also fill you up in a way that no carbs or junk food possibly can, because of the hormonal effects of carbohydrates, which stimulate hunger. When you've just eaten a big steak, you're FULL. The very thought of even the most delicious s'mores sounds bad, because you simply are not hungry.
3. Make substitutions. For example, instead of eating a Snickers bar, develop a taste for really dark (like 80%) chocolate. It will taste bitter at first until you get used to it, but it's much better for you, you won't feel compelled to eat as much of it because the taste is so much stronger, and it lets you satisfy that "chocolate" urge, if you have one. After awhile, as you break the carb-addiction cycle, it will be easier and easier to resist the temptation to give in. If you're at a fast food joint with your family, instead of getting a burger, fries, and a big coke, get a big burger with cheese, bacon, no bun or veggies, and unsweet tea or water. Optimal? Probably not. But it will fill you up and will be a heck of a lot better than giving in and having those fries, that coke, and maybe an ice cream sundae too. Sometimes perfection is the enemy of "good enough," and it often happens that if we strive too much for the former, we achieve neither.
4. The most difficult part of a diet is the social aspect. It's embarrassing not to be able to shamelessly indulge in everything our friends do. It's awkward having to decline a piece of birthday cake at work. Keep this in mind and make whatever prudent changes you can to avoid "occasions of sin" -avoid being put in situations where you know the likelihood of being tempted to eat junk is going to be high.
*Edited to add:
5. Clean your kitchen and refrigerator. If it comes in a box, chances are it's not good for you. 90% of the edible, nutritious stuff from the store, the real food, is found around the edges -the meat, eggs, cheese, and fresh produce sections. The stuff in the aisles in fancy packaging is generally all overprocessed, chemical-laden junk. Don't go to those aisles, and clean out your kitchen cabinets, which, if they're anything like mine, were full of that sort of stuff. If you have it in your kitchen, it's available to eat easily, and so in a week moment you will eat it, and then regret it. Throw out what's expired and donate the rest to a food pantry.