The book and the movie are very similar, there are only a few things that change, but to be honest is difficult to see the difference because it is almost the same, the things that change are some scenes, one of them is when in the book Augusts Water's parents do not let him take Hazel downstairs unsupervised to watch V for Vendetta, instead in the movie, Gus and Hazel go downstairs without any parental guidance.
Another different thing is that John Green writes that Hazel goes to visit Isaac at the hospital following his eye surgery, in the movie she doesn't.
It's frustrating that many of the details in the book that really make it so much better and all, especially extremely important details, are left out of the movie! I don't like that they changed so much of the details either. I liked the movie, but the book is a million times better. You can't say you're a fan unless you've read the book.