Replacing white, bleached flour with whole grains is a high impact healthy change. Replacing sugary soda with water is another (a 20oz coke has almost 16 tsp of sugar in it!). Snacking on fruit or vegetables (apples, pomegranate, blueberries, carrot sticks, etc.) and almonds instead of chips and cookies is another.
Some big healthy changes to get started without significant alteration to your meals!