• Realise you have a problem.
• Educate yourself, so you know what to eat and not to eat.
• Start to pay attention to what you eat.
• Eat more whole foods, and plants that have not been processed.
• Eat lots of fibre and protein, but remember no food is healthy if you eat it excessively.
• Pick healthy snacks like fruits and nuts instead of eating sweets.
• Minimize your intake of saturated Fat.
• Eat healthy fats like avocado, nut and seeds.
• Eat complex carbohydrates (pick whole grain bread ).
• Eat less simple and refined carbohydrates (like white bread).
• Drink lots of water - a minimum of 2 litres a day.
• Reduce your intake of salt.
• Make sure you don't have a vitamin deficiency. If you are struggling to eat different types of food or you are an extremely picky eater you can take supplements, to make sure you get the vitamins you need.
• Eat less fast food. If you are going out, try to eat a meal at home, so you won't end up buying fast-food.
• Cut out foods that you may be allergic to like soy, milk, gluten and eggs.
• Limit your intake of caffeine from coffee, soft drinks and tea.
• Make sure to eat three meals a day (breakfast is the most important meal of the day)
I choose to interpret the question different in the rest of my comments though. Being healthy isn't all that important. You could die of a car crash tomorrow so why worry about that extra slice of cake or jog you thought about going for? Really even with fair to poor health you can live for quite a time and enjoy life, no need to bother with GOOD health.