One thing you may want to know before you start using Goodreads is: "is the website free to use?" The answer is "yes, for the most part." It costs you no money to do the following things on Goodreads:
Create an account
Rate and review books
Add books to your virtual bookshelves
Search for books
Read certain e-books
Join groups of people who enjoy certain types of books
Post a message in a discussion forum about a book or book-related issue
Do other fun things such as take quizzes, participate in giveaways, and sign up for newsletters
In fact, there are really only two things that you can do on Goodreads that will cost you money: purchase books and advertise.
Purchasing books on Goodreads
Goodreads doesn't sell books itself; instead, it directs people to retailer websites, where they can purchase books. Some books will have previews on Goodreads; if you like the preview, you can proceed to a retailer website and purchase the full book. Goodreads gets a cut of any sale made this way.
Advertising on Goodreads
If you're an author on Goodreads, you can place advertisements on the website to promote your book. It's a cost-per-mention system, so the more people who click your link, the more you have to pay.
For more information on advertising with Goodreads, click this link.
Anyway, that's a little bit of information on what's free or not free on Goodreads!