A journal response is a kind of reflection on something, in this case a book. I am not sure how long of a journal response you are expected to write, or whether you are expected to respond to the entire book or just a small section of the book. But generally, there is so much to say about a book, it is a good idea to limit yourself to just one small aspect of the book.
When you do respond to one small aspect of the book, you are usually expected to state your thoughts and feelings and to make connections between yourself and the book. What did the book make you think about? Is Pi like you in any way? Is he unlike you in any way? Is there an episode in the book that struck you as important in any way?
A journal entry does not have to be as formal as an essay, but it should have a little structure, I think. For example, you should have some sort of main idea stated in your entry and stick to the that point as you write.
If I were writing a journal entry on this book, I might write about Pi's interesting approach to religion, his idea that it is fine to have three different religions, all at the same time. So I might start out a journal entry with a sentence like this:
The idea that it is possible to have three differerent religions, as Pi does, is a useful idea in today's world, when we are so divided by religion.
Do you see how I am setting the stage to reflect on just one particular aspect of the book?
Choose an aspect of the book that you have something to say about, write a statement, and then keep going. It is much easier than you think!