Excellent question but one that is difficult to answer.
The best protection is sunscreen. For most people a sunscreen with at least SPF 15, preferably SPF 30, will be sufficient. Technically the SPF (sun protection factor) measures a sunscreen's ability to protect from skin turning red. In general, this translates into protection from UV-B radiation, not UV-A radiation. Different protection systems are being devised to classify materials for their protection against UV-A radiation, less important but still important in protecting against skin cancers.
Another important concept is reapplication. Be sure to reapply sunscreen throughout the day. This includes when you're not necessarily in the sun but performing daily activities indoors and outdoors.
Of note, most glass protects against UV-B radiation, but not UV-A radiation, so there is still a sun exposure risk. For patients with risks factors for skin cancer including light skin, previous skin cancer, and family history among others, higher rated sunscreens may be necessary and a physician's opinion should be sought.