Clear windows are critical to driving safely, but it can be a challenge to keep them from fogging up in the winter because of moisture in the car. The goal is to remove the moisture.
For a quick fix use the “Defrost” mode. This will automatically engage air conditioning and a high fan speed, directing warm, dry air to the windshield and often the front side windows. This can heat the glass to aid clearing snow and ice while removing condensation from the inside. As visibility returns, you can turn down the noisy fan.
Air conditioning is the key. Cold air may be the last thing you want in the winter, but remember, this mode isn’t called “chill”—it is “air conditioning” because it dehumidifies the air. You can still set the temperature to a comfortable level and enjoy warm, dry air from the vents.
If you are manually adjusting the climate control system, rather than just using Defrost, be sure to keep the system in fresh-air mode, allowing outside air to enter the system. (This is the opposite of Recirculation mode, as indicated by the symbol of a car with a looped arrow within it. Recirc is great when following a filth-spewing diesel truck or passing by a stinky skunk, but it will make the fog condition worse.) Sometimes cracking the windows open slightly, especially if the car is full of passengers, may be all the conditioning you need.
Resist the urge to rub a cloth on the windshield, because that can lead to streaks that can obscure your view in direct sunlight or possibly when driving toward headlights.