My dermatologist advised me to wear at least a SPF30 every day and SPF50 if I plan to be outside most of the day. Make sure you use enough sunblock as well. The recommended amount is about one ounce which is about a shot glass. If you use less then you are not protected. Also, as a general rule you should reapply every two hours.
Cera Ve' is my favorite:
Available at any drugstore for around $11. My skin loves this stuff however, keep in mind everyone's skin reacts different to products so you have to find the best for your skin type.