You have to learn to crawl before you learn to walk. Bodies do not like change, so you have to gradually "trick" it into being healthy. Start small, like taking a short walk three times a week, or adding more veggies to your diet. Over time start building and adding more healthy habits.
Note: It takes about 2 weeks to form a good habit so stick with it! If you can't stick with it, try something simpler. Remember, Baby-Steps!!