How should I react to someone saying, "Good luck," to me? I don't believe in luck. I feel God is in control of everything, and chance doesn't fit into the equation.
One approach could be to thank the person for his or her concern and then briefly explain that “luck” might be for other people, but you go through life with God at your side: he watches over you and works everything for your good (Romans 8:28).
Your question illustrates how Christians can find witnessing opportunities in the littlest of everyday conversations. God bless your witnessing!