As Insertjoke has indicated, this really isn't the easiest form of divination. Crystal balls, black mirrors, bowls of water, and like tools are used by gazing into them while in a deep and receptive meditative state. You "see" images reflected in them, but they are much like the visualizations used in magick or how most people perceive ghosts, and it's not actually seen with your physical eyes.
You certainly can begin with gazing forms of divination if that's what you're drawn to, but since actual crystal balls are very expensive I'd suggest starting with a bowl of water or a false black mirror (you can make false ones yourself using black paint and a picture frame, and they work just fine). But if you don't have much practice with divination I would suggest begriming with a form of divination like runes, tarot, or even a pendulum.