WIndshield wiper fluid has additives in it that prevent streaking on your windshield. They also increase the lifespan your the windshield wiper blades.

1. Locate the windshield wiper fluid reservoir - it will have a small picture of a windshield wiper on the cap. 
2. Remove the cap and a long plastic indicator should come out with it, much like when you check your oil level. 
3. There are two ways to check wiper fluid level: by the minimum/maximum level indicators on the side of the reservoir or the plastic level indicator attached to the cap.
4. Use one of the aforementioned methods to determine how much more fluid you need to add to the reservoir.http://www.youtube.com/watch?v=Nmy-B8DtbMg&feature=player_embedded 

5. Use a funnel to avoid spillage and add fluid to the reservoir.
6. Remove the funnel, replace the cap and you are ready to go!http://www.youtube.com/watch?v=Nmy-B8DtbMg&feature=player_embedded