The easiest rule to a healthy lifestyle is to only eat real, whole foods. That means like foods that don't come already made, or in a box. Just unprocessed, REAL food, like fruits, vegetables, meats, nuts & seeds. Some minimally processed stuff, like whole grain breads & cereals, is okay in moderation. Beware of boxed "healthy" foods they sell at those organic food markets, those are mostly scams capitalizing on ppls desires for miracle products.
One easy beginner rule is to just cut liquid calories. They're the worst offender in most unhealthy diets. Many people could simply lost a few pounds making that ONE change in their diet. Liquid calories are just too easy to consume too quickly. If you wanna eat junk food, at least EAT something you can chew on abs really enjoy, instead of gulping those calories in a few seconds.
That means only water, unsweetened coffee & tea, and other zero calorie beverages. No juice, no 600 calorie caramel machiatto from the overpriced coffee shop, and no soda.
Look for naturally flavored seltzer if you're really into soda, or for a short time switch to diet /zero calorie soda (but it still should be considered very unhealthy) If you're exercising regularly, a protein shake is maybe an okay exception to the "no liquid calories" rule.
Also, as a beginner rule, cut down on anything with added sugar. Your true enemy is sugar, not fat.
One way i get started after eating too much junk for too long is give myself a few days where i allow myself to eat unlimited fruits & vegetables, and a moderate amount of nuts. Later, i cut down on fruits because there is just too much natural sugar, but for the transition from junk food with added sugar, to stop the cravings, i eat any delicious fruits i want whenever i get a craving.