Exterior windows and screens build up dirt and dust over time. Not cleaning these can lead to more dust inside the home when windows are open, and can lead to weep drainage holes in the window frame itself to get plugged up. Over time this can also deteriorate the operation opening and closing of windows. It is important to maintain the appearance of your home for the benefit of the overall neighborhood value of homes, and to keep the home healthy when it comes to air flow inside the home.
In doing this maintenance tasks, you are going to need a bucket with a cleaning solution, a cloth side of a squeegee to clean the window, and the rubber side of a squeegee to wipe the windows clean. Depending on the height of the windows, you will need a telescoping handle to reach the windows, and you might even need a ladder if you have a two story home. Be sure to take proper ladder safety precautions.
Prior to starting, it is also a good idea to take off all the window screens and hose those down. Then put your cleaning cloth with a flat edge in the cleaning solution. Scrub the window up and down. Then take the rubber squeegee and in the same vertical direction, drag the squeegee from the top to the bottom.
Some people will also considering hiring a professional to come in and clean all of your windows inside and out.
Here is a great video from iScaper on how to clean your windows.