Answered[ Edited ]
SPF http://www.sephora.com/sunscreen-skincare - Sunscreen is not all day protection. It will need to be reapplied as directed. Even sooner when engaging in water or physical exercise activities.
To detract early signs of aging, an SPF of 15 or higher PA+++ or that protects against both UVA and UVB rays is recommended. For more fairer skin types; go SPF 30 PA+++ or above.
PA+++ is the easiest label to recognize. It protects against both UVA and UVB. The more pluses (ranges from 1 to 3 plus symbols), the more protection against both kinds of rays.
Broad spectrum is another indicator but the FDA is doing away with that label as it has proven to be misleading on some brands.
Clothing - Hats, sunglasses. There are some UV protectant clothing lines and brands out there as well.
Other resources: The FDA official website, Consumer Reports. Should find information looking up keywords FDA sunscreen _or_ consumer reports sunscreen in the search engine of choice.
Note: SPF is all year round. Especially in higher elevations (high chance of being sunburnt while skiing!) or year round humid/tropical climates. Even on cloudy days, it is possible to get sunburnt (been there done that; not fun).
I've found that keeping skin moisturized is the key to healthy skin. Obviously, protecting skin with an SPF is vital. However, it seems as though I avoid sun burns when my skin is totally hydrated. My trick: apply a great moisturizer (i LOVE C Booth's Vanilla Bean lotion) right after you get out of the shower while skin is still damp, and also right before I go to bed. But remember...SPF is super important so don't forget to apply that prior to sun exposure and continue to re-apply every couple of hours! Also, be sure to choose a moisturizer that has an SPF in it, as well as your lip balm. These days, some foundations (liquid & powder) even include SPF.
Andddd I almost forgot! Drink TONS of water while in the sun, and listen to your skin. When you're skin has had enough sun, it'll let you know. Stepping out of the sun and taking a break will be worth your while!