Windshield cracks often begin with a chip in the glass. This tends to occur when another vehicle kicks up a stone that flies backward and hits your windshield. If you work on this chip right away, sometimes you can stop the chip from turning into a crack, or stop a small crack from spreading further. This might only be a temporary fix, but it can buy you some time until you save up enough money to have the windshield replaced.
Put the car in a warm garage if the weather is cold. Start with a completely dry windshield that's somewhere near room temperature. Dry the windshield if necessary with a clean cloth or hair dryer.
Buy glue that can stop the crack. Glass shops and auto parts stores sell windshield repair kits containing a specific type of glue. If you only need to fill a chip to stop a crack from beginning, you also can use a high-performance adhesive like Super Glue or Crazy Glue.
Fill in the windshield chip with glue.
Insert glue into the crack. Follow directions on the kit package. Use the syringe-type applicator that comes with the kit. Begin at one end of the crack and fill it with adhesive. Wipe away any excess glue with some rubbing alcohol applied to a clean cloth. Move slowly along the crack until you've filled the entire length with glue.
Allow the glue to dry for at least an hour and preferably for a few hours before driving any distance.