You can get your plant to re-bloom. It takes some work, but it can be worth it, particularly if you've gotten some of the newer, more expensive varieties. Continue watering and in May you should start seeing strong growth. You may have to move the plant to a slightly larger pot. Keep the plants in indirect sun and keep watering.
Poinsettias are photoperiodic, which means they get their color according to light.
To create color for the Christmas season, you must be careful in your timing. Starting on October 1, keep them in total darkness 14 hours each night. You can do this by moving them to a dark room or covering them with a big cardboard box. Your garage won't work because it gets opened and light comes in. They must also have 6-8 hours of bright sunlight each day.