Pop the hood, look for this symbol

Open it up. If you live somewhere it doesn't freeze, you can fill it with water (distilled will keep minerals from accumulating on sprayers, but tap is better than nothing). If freezing is a possibility, fill it with windshield washer fluid, usually available at any gas station. Reclose the cap.

The last time I showed someone this was in a rental car. She was driving and got mud splattered on the windshield. Panic ensued as the wipers just smeared the mud and generally did not improve the situation without fluid. So we make it to the closest gas station and take care of her and the vehicle and showed her how to do it. Driving was much better being able to see. Anyway, just one of those things that isn't important until it's very very important.