First, consider what you want to achieve. Crystals can be used for everything from attracting love to relieving stress. Once you have a goal in mind, you can narrow down your choices.
Next, take into account the energy of the crystal. Some crystals are very high-vibration, while others are more calming. Consider what energy you need before making a selection.
Finally, trust your intuition. Crystals emit energies that can be felt on a physical and emotional level. When you hold a crystal, pay attention to how it makes you feel. If a particular crystal speaks to you, it is probably the one you need.