To summarize any text, you want to zero in on the important details. If you are summarizing a work of fiction, pay attention to things like theme, characterization, character development, and plot progression. Below are some ideas to help get you started.
R. J. Palacio's Wonder is about a 10-year-old boy named August who grew up with a severe facial deformity due to a genetic disorder. Though August had been home-schooled throughout his earlier years, he decides to start going to school in the fifth grade and starts attending the private school Beecher Prep. The novel recounts his problems, especially problems with friends, and how he matures through it all.
Part II of Palacio's Wonder is written from August's sister's perspective, Via, short for Olivia. Part II especially focuses on developing Via's own problems, problems derived from her own circumstances as well as problems derived from her brother's problems. Via starts Part II off by explaining that she has known since the age of six that August's needs would have to come first and that her "worst day, worst fall, worst headache, worst bruise, worst cramp, worst mean thing anyone could say has always been nothing compared to what August has gone through" (p. 60). She also explains that she didn't feel she understood these things out of a noble part of herself; she simply understood how things worked. In this part of the book, she also talks about her two best friends Ella and Miranda and how much time the three of them spent with August. In particular, at this time in her life, she is starting high school, and one of her problems involves her friends. On the first day of high school, she sees that both Ella and Miranda have changed and become distant. They even start hanging out with the popular group, leaving Via to make a new friend named Eleanor. Via starts hanging out with the smart kids and finds herself developing a new crush on quiet, violin playing Justin. The part also recounts conversations with her brother, paralleling her own friendship problems with her brother's friendship problems. However, by the end of the part, August tells Via that Miranda phoned one day just to speak with him, ask him how he was doing, to say that she'd always be like a big sister to him, and to tell via Miranda said hi. Needless to say, Via finds this information very heartwarming.