Depending on the type of vehicle you own the wind shield washer reservoir will be located in different places however it is mainly located in the bonnet/hood of the car it will have picture of a window wiper on it this must be refilled, is is usually a white plastic colour.
On most cars, the windshield washer reservoir is located to the left of the radiator. Some cars, especially V-8 models, move them to the back because of space constraints.
Accessing the washer reservoir is rather simple. Pull the hood catch inside the vehicle. Usually under dash on driver's side. Some vehicles have the hood catch on the outside of the vehicle usually at the front of hood. Raise hood. You will find a reservoir, plastic with a windshield with wiper
logo on the lid.