You probably have already dropped the over processed & junk foods..and read labels of what you do buy..

If not, that would be a first step to really clean eating, and being very aware of what you buy.
More fresh foods , less pre-packaged, less fried foods, more baked, boiled, broiled, steamed or slow cooked.
Good fats vs bad fats..
http://www.whfoods.com/foodstoc.php

Of course it's hard to do 100% of the time.