Get some bits mad for glass like the guy above said. Sometimes when you drill in to very slick or smooth surfaces, it's better to cut out a piece of wood and fit it to whatever you're trying to drill. Drill through the wood, and by the time you get to the glass, the wood will have your bit stablized and it will be easier to enter at the angle you're trying to get. Also, you may have to go slowly at first, or the whole time for that matter. There are different ways, but you can do it this way your self