Always wear sunglasses that block 100% UV when out in the sun. Buy the sunglasses that cover your eyes from sides as well. UV rays get reflected from the ground surfaces as well. UV rays harm the eyes and cause cataracts.
Cover yourself up when out in the sun. Wear thicker fabrics like denim instead of cottons or wool, which let the UV rays pass through them much more easily.
Wear sunblock with SPF 30 or more at least 30 minutes before heading out in the sun. Avoid putting it on skin above your eyes/forehead.
Wear a hat to protect head and skin around your head/neck.
Drink plenty of fluids to keep yourself hydrated. Water is the best. How can you determine if you're dehydrated? An easy indicator is the color of urine. Clear means you're well hydrated. Don't drink too much water all at the same time. That could cause problems.