What is the First Thing to Do After Becoming Lucid?
This question cannot be separated from "how can I lucid dream?" Many people unexpectedly become lucid once or twice during their lives, but the event elicits such awe and excitement that the dreamer exceeds some sort of emotional threshold, triggering an awakening from sleep. So the first thing to do immediately AFTER becoming lucid (such as after asking and testing the critical question) is to STABILIZE!
Stabilization occurs by performing one or more of several techniques: spinning, hand-rubbing, affirmations (i.e., "Lucidity Now!," or "Increase Vividness!" - sounds silly but works). When done right after becoming lucid, stabilization will increase the clarity of your perception in the dream and prevent a premature wake-up. As you feel the dream fading, or lucidity decreasing, periodic or continuous stabilization may be needed. The longest lucid dreams can occupy as much as a whole REM-cycle allows, and the last REM period of the night can run 45-minutes. That means that a lucid dream could potentially last 3/4-hour. If stabilization is maintained.
If you find yourself pre-lucid, wondering whether or not you are in a dream, then the first thing to do would be state-testing, or reality-testing. This is done by finding some text to read (don't worry, something will be around to read, even if it is just one word), look at a word or a sentence, look away, and look back. If you do this twice and nothing changed, you are awake. If the words change (when you know they shouldn't), you are in a dream. Or, look at a digital clock and do the same kind of thing as above. Either way, if it turns out you are in a dream, stabilize. After that, if you have a dream-goal (an "experiment"), continue on to that.