You've read about the importance of sunscreen — either on the bottle of SPF or this very website — but it's true: sun exposure is the number one cause of premature ageing. That's why you need to protect your skin from the harmful effects of UV rays (which include but are not limited to dark spots, fine lines, and skin cancer, to name a few) early on. There are dozens of great sunscreens out there depending on what you're looking for.
If you're a naturalist, maybe you prefer a physical formula, which includes actives like titanium dioxide or zinc to physically block the sun from penetrating the skin. Or, if you like to stay active and want something that'll absorb into skin, chemical sunscreens are a good way to go. Either way, any SPF will keep you safe and reduce any future damage.