This quote is on page 122 in my version of the text.
If you have a different version, you can easily find this quote by heading to Chapter 8, then flipping to about halfway through the chapter. Look for the break in the text after the phrase "twenty minutes" and before the sentence that starts with "I stayed in the back yard." That's when the scene begins that features this quote.
It's the scene in which Augustus says to Hazel:
"You realize that trying to keep your distance from me will not lessen my affection for you."
It happens when Hazel realizes that her cancer will severely shorten her life, and she doesn't want her eventual death to inflict pain on Augustus, her new boyfriend. But he wants to assure her that he likes her very much anyway and is willing to risk the pain of losing her.
How did I find this quote? Not by flipping through the entire book, looking for it--that would be tedious and time-consuming. First, I Googled the quote to see if anyone had already discussed it and if so, if its location within the book was mentioned. That worked: someone had blogged about that quote and included the page number. If for some reason that strategy didn't work, then I would try thinking through the plot a bit, trying to decide when someone would have said this to another character. It would probably be after Hazel and Augustus started dating, but before they got too serious. So I'd start looking through the middle of the novel until I found it, which would also work. But perhaps the most efficient strategy of all is to get your hands on a copy of the ebook, and use an automated search function to locate the quote. (Search for a small, unusual phrase within the quote, such as "lessen my affection.")