Choose A Bulb!
Amaryllis bulbs are classified by size, which is determined by measuring around the outside of the bulb at its widest point. Generally, the larger the bulb, the more flowers the plant is capable of producing.
Make sure it is heavy and firm—not moldy, squishy, or injured. It should have some fairly long fleshy roots attached, too.
If you can’t put the bulb in a pot right away, store it in a cool, dry, dark place until you can.