Though many sunscreen bottles tell you not to use it on babies under six months, the American Academy of Pediatrics recently said that it's okay to put a little sunscreen on your infant if you have to. Before you slather it on, though, put it on a small bit of baby's skin and watch for any reaction. In an ideal world, baby's virgin skin wouldn't get any direct sun exposure. Realistically, at some point you'll probably find yourself in a setting (the beach, a picnic, a hike) where catching a few rays is inevitable. In these situations, slather the parts of her skin that might be exposed with sunscreen. Find one specifically for babies, and apply thirty minutes before you head outdoors and again every two hours.
Your first defense, though, should be true sun blockers like long-sleeved tops, pants, and a hat (look for one with a three-inch brim). Keep her under the shade of an umbrella, tent or stroller as much as possible. Don't ever bring a tiny baby outside for too long on a hot day — their bodies can't tolerate extreme temperatures well.