Glass can be painted with acrylic paint. There are some paints that are specifically made for glass, but regular acrylic paint works as well. A varnish is needed to glaze over the work.
Wash the glass in warm soapy water, then rinse it in clear warm water, and let it air dry. Dab a paper towel with rubbing alcohol or white vinegar, and use it to wipe down the glass, removing any soap residue. Let it dry for 15 minutes. Follow the directions for painting. Apply a coat of varnish over the finished work after the paint is completely dry.