How can I buy a book on Google Books?

I'm interested in purchasing a book from Google Books, but I'm not sure how to go about it. Could you walk me through the process of buying a book on Google Books? Is there a specific app or website I need to use? Do I need to create an account first? Are there any payment options available? And how do I know if the book is available in my country? I appreciate your help in guiding me through this process.