The first thing is, try to stay out during the heat of the day. The worst times tend to be from 11am-3pm. If you have to be out during the day, stay covered. Lighter colors are better than darker. Wearing a hat can help keep the sun off of your face.
When outside, wear sunblock, especially if you are fair skinned. Do not forget to reapply it within the time frame recommended on the bottle. The more skin damage one sustains throughout their lifetime increases their risk of different forms of skin cancer.
Drink plenty of water. Our bodies are made up of 60% water. During the heat of the day, our bodies cause more water to evaporate. This increases our need for water.