A good early step towards an improved diet — avoid foods containing ANY of the following: antibiotics, hormones, GMOs, pesticides and slaughterhouse waste. This may seem like a big action if you are not already a vegan eating primarily organic foods, but it is actually relatively easy.
If you are choosing organic foods now, you are already covered to a certain extent. The genetically-modified crops are cotton, soy, canola, corn, Hawaiian pineapple and squash, so these are the ones to watch out for specifically. If you eat meat, it should be from free-range animals (warehoused animals are fed GMO-laden grains); if you eat eggs, buy those from ‘barnyard scratch’ chickens. If dairy is still in your diet, purchase organic brands with labels marked as GMO- and antibiotic-free.
Raw foodists have to be alert, too. Certain foods, such as berries, greens and celery, should always be organic. Other foods such as onions and avocados are not as susceptible to pesticides – either because they have natural pesticides, or because the common pesticides used on them are not systemic and the peel is protective.
Until you are familiar with all the safe food choices, you might want to refer to the shopping guides provided at http://www.nongmoshoppingguide.com for Non-GMO foods and http://www.foodnews.org for pesticide information. Just note that a food item might be considered safe in one respect while not safe in another. (Corn, for instance, may be without excessive pesticide residue but still be grown from GMO seed!)
But not to worry — grocery shopping becomes second nature before you know it!