Choose only sunscreens labeled as "broad-spectrum." which blocks both UVA and UVB rays. For UVA protection look for avobenzone or oxybenzone as ingredients. Avobenzone degrades in sunlight, so re-apply it frequently.
Don't rely solely on SPF (sun protection factor) numbers to guide you. SPF only measures UVB protection. Choose at least SPF 15, but higher is better. Re-apply it frequently, every 2 hours or more if swimming or sweating. Research shows that products often give less protection in sunlight than their SPF numbers suggest.
An alternative to sunscreens are sunblocks which provide a physical barrier between your skin and the sun rays. Look for sunblocks containing zinc oxide or titanium oxide.