Sunscreens are by no means adequate protection against the sun's skin-damaging ultraviolet radiation. Keep in mind that sunscreens are simply one part of an overall skin cancer prevention strategy.
The other pieces, of course, include a healthy diet, limiting your exposure to the sun, and wearing protective clothing, hats and sunglasses when outdoors.
It's important to find a sunscreen that protects from both UVB and UVA radiation. Ideally, find one that offers a sun protection factor of 15 or higher. Apply evenly to all exposed skin 30 to 60 minutes before going outside, and reapply every two hours, more often if you're swimming or during strenuous exercise or activity.
Be sure to check the labels for chemicals that may cause allergic reactions, or test the sunscreen by rubbing a small amount onto the skin of your upper, inner arm for three to five days.
Natural products stores offer sunscreens with polyphenol antioxidants found in extracts from green tea and grape seeds, which have been shown in some studies to reverse sun damage.