The quote is in Chapter 5, from Erich Maria Remarque's All Quiet on the Western Front. The page number will depend on your edition of the book. In the Penguin Random House edition, that particular quote is on page 67. The character being quoted is protagonist Paul Bäumer, a soldier in the German army. Remarque's book is unique in the canon of war literature because it does not attempt to romanticize war. Instead, his work describes the devastating and traumatic effects of war on young men, in particular the generation that fought in World War I.
In Ch. 5 Müller, another soldier in the German army, is questioning his comrades about their post-war plans. The younger men can scarce imagine a life beyond war. Their answers are mere fantasies about drinking and women. They stand in stark contrast to the concrete plans of middle-aged Stanislaus Katczinsky, or Kat as he is nicknamed. He has already built a life for himself outside of war, and he intends to return to his family. Paul, on the other hand, can not even begin to conceive a plan. He was so young when he joined the army that he had not started his life. As a result, he doesn't know how to live. Throughout the story, Paul sadly contemplates the mental toll of warfare.