To answer this question, you will need to look over the chronology of part 1.
When the story opens, we meet Guy Montag, a firefighter, who is just finishing a shift. As he walks home from the fire station, he meets with Clarisse, one of his neighbors. Clarisse is not like anybody else Montag knows; she has a strong sense of curiosity and asks him profound questions, including whether he is happy.
It is after parting with Clarisse that Montag returns home to find that Mildred has overdosed. At first, he thinks she is just sleeping, but once he sees the empty box of pills, he knows what she has done. He immediately contacts the emergency services who send two men to pump her stomach.
Both of these events have a profound impact on Montag because they prompt him to question his role as a fireman and the nature of his society, more generally.