This book has several really great themes that can be supported with details from the story itself. If you pick a theme and want to explain it well, the best way to do this is to figure out exactly what the author, through her novel, is saying about that theme. For example, one of the themes of the book is friendship. That's pretty easy to pick out; Sophie, the main character, has two really great friends and enjoys her relationship with them. However, to be more detailed, just exactly what role does friendship play in Sophie's life? In looking at the novel itself, Sophie has a hard time with boys and with her parents. And every single time she is depressed or struggling with these issues, her friends are there to cheer her up and help her to get through them. Specifically, after she breaks up with the dangerous Chaz (who she met online), her friends take her out for a fun night on the town. The evening cheers Sophie up and helps her to get over her sadness over her recent break-up.
So, whatever theme you might pick (see the link below for more ideas), ask yourself, "what role does that theme play in the novel," or, "what does the author seem to say about that theme"? Once you have that answer, then look for supporting details, from the book, that show that message being played out. I hope that those thoughts help to get you started! Good luck!