I need help with writing a response paper for the book/mini-series Testament of Youth.

Your response will depend on which of the book’s themes you identify as most important. In any film or television adaptation, there are always differences from the source material, so be alert for any changes the filmmakers made.
Vera Brittain’s memoir has long been considered a powerful statement about the devastating effects of war that also is a highly personal story of her individual experiences. You might be moved by the war-related aspects of the book. The book has earned a firm place in the World War I literary canon, among other works like All Quiet on the Western Front and A Farewell to Arms. Your response paper could focus on some of the themes of war—both the experiences Brittain has during the war and its effects on her, and her boyfriend Roland, as members of the “lost generation.” Does her despair at seeing “nothing left in the word” continue, or does she regain hope?
Brittain, of course, differed from the men who wrote about their war experiences, in that she served as a nurse. Her decision to serve was not what she expected to do with her life. She has interested modern readers in part because she was a feminist, and before the war, she had already started university, which was not what her family expected or wanted. The ways that her experiences in war affected her progressive principles would be something to look for in your reading. You could think about how becoming a writer intersected with her earlier life goals.