Clarisse's impact on Montag is, from the very start, profound and immeasurable. Before he even meets her the first time, he is moved by the mere fact that she has been in the area. Bradbury writes that as he walks home, before he even meets her, he senses a change in the air, as if "charged with a special calm." He senses breathing, an energy, a force, and all of this foreshadows the power that Clarisse will have over his life; he can feel it even before he has met it. So, Bradbury uses foreshadowing on Montag's senses to indicate the coming meeting with a very important character.
Then, when he sees her for the first time, Bradbury gives nearly 2 full paragraphs to describe her intensity and beauty. He states that Montag felt "he had said something quite wonderful" when she looked at him, and he tried to say hello but couldn't get the noises out of his mouth right away. In short, he is immediately moved by her; speechless; impacted in a profound way, before they even talk. As they talk, he is reminded of a wonderful childhood memory of total happiness with his mother--Clarisse moves him to think of happier times, and of fond memories he had forgotten. Then, after that first night, his entire universe is shaken as he realizes quite clearly that he is not happy. She had asked him that question and he had defended his happiness, but upon arriving home, her words drove him to admit that he wasn't. He ponders her impact on him in such a short time:
"How long had they walked together? Three minutes? Five? Yet how large that time seemed now. How immense a figure she was on the stage before him."
So, to put it simply, Clarisse immediately has an immense impact in Montag's life; in fact, a life-changing influence. She is a catalyst that starts all of Montag's truth-searching, that eventually leads him to leave behind everything that he thought he believed in pursuit of real happiness. I hope that helped; good luck!