The advantage of Goodreads is that how you review is largely up to you. Goodreads will only delete a review in very specific circumstances. According to their review guidelines they will only delete reviews that are off-topic, personally attack the author or other reviewers, are plagiarized or are trying to sell products. If you aren’t doing that your review should be acceptable.
Goodreads is about helping people choose to read or not to read certain books. Reviews should give enough information about the book and your feelings about it to help someone who is not familiar with the book decide if they want to read it or not. If you are concerned about spoilers, there is a box you can check at the bottom of your review before you publish it that says “hide entire review because of spoilers.” When you publish your review with that box checked there will be a note at the top of your review that says it contains spoilers. That way people can choose whether or not to read it.
Reviews on Goodreads generally consist of two parts. First, they contain a synopsis of the book. Think about what might be written on the back of a book or on a book jacket. It should explain enough about the book to help people decide if they’d be interested in it without giving the whole story away. Often that means telling the story up to the climax or high point and then stopping so no one knows how it ends. If there are key details that make the story special or unique (the name of the killer, a twist no one saw coming, etc.) they should be left out. But setting, basic character info, what kind of story it is (comedy, drama, mystery) and a quick statement of plot are helpful.
The second thing to include is an honest review of the book. Explain what you liked and didn’t like about it. Goodreads values honesty in their reviews; that is the point of the site. Knowing the plot of a book does not always help us know if we will like it. Reading what others thought of it and why gives us more information. So give honest feedback on how you felt about it and why you felt that way. Did you like the story? The writing? Did it meet your expectations? Did you think it was well-researched? Was there content you did not appreciate? Would you recommend it or not? These are the kinds of questions to consider when you write your review.