With a large, sharp knife, cut off the bottom of your bunch of celery about 2 inches from the base. You will be using the base to grow your new celery and storing or using your celery stalks.
NOTE: According to Cook's Illustrated the best way to store the stalks until you are ready to use them is to wrap them in foil or plastic wrap and refrigerate them. They did find that the foil wrapped celery remained fresher a bit longer than the plastic wrapped celery. To revive stalks that have become a bit tired, they also recommend cutting a half inch off both ends and putting them in an ice water bath for thirty minutes.