Have you tried the Frequently Asked Questions page? That will give you a good idea on how to start. Question 134 (in the Releasing and Journaling section) might be particularly helpful. http://www.bookcrossing.com/faqs
I see that you have already registered a book. To create a new crossing zone, you must first start the process of releasing the book.
To release a book, start by selecting the country, state and city/town you want to release it in. The next step will be to choose a release zone. If the place you want to release the book is not already listed, there is an option to "Add a Zone".
Remember, if the zone in which you want to release the book is already listed, you don't need to add it again. That just creates an unnecessary duplicate. Just click on that zone and carry on following the prompts.