Every book is different and there can be no set formula for a review, but please bear the following points in mind:
Ideally online reviews should be about 400 - 600 words long: shorter is better — it's more likely to be read. If you quote the book, please keep the quotes fairly brief and always give the page number.
Try to summarise what the book's about. If you can do this in one soundbite sentence, even better!
Assess whether or not the book achieves what it's aiming for: try to identify strengths and weaknesses — what, if anything, is missing?
Is it well written? Does the language flow or is it too dense or stilted to be readable?
Keep the genre in mind: if it's fiction, for example, don't give away the plot!
Offer constructive criticism: imagine yourself at the receiving end of your review — how would you take it if someone said that about your life's work? Engage with the author, if it's atrocious, say so, but be gracious about it. There's a fine line between critiquing an author's work and personally attacking an author: please tread carefully.
Find fault by all means — but that's not your aim: your aim is a fair and honest appraisal that will help prospective readers decide whether or not this a book they want to read.
Try to include a sentence or two, ideally towards the end, to identify the target readership — something along the lines of "If you're concerned about/enjoy/ready for... blah blah... then this is the book for you/essential reading" — or one to avoid, as the case may be!
Please use inclusive language throughout your review unless a comment is intentionally gender-specific. For some discussion of why this is important, please refer to this Author—Reviewer Dialogue.
Finally, remember that a review is your opinion: by all means be objective, but don't be indifferent — make it personal: say what the book does, or fails to do, for you.
Your First Review? If you're submitting your first review, please include a sentence or two about yourself to help readers understand where you're coming from; this will be included at the end of your review and in the Reviewers Index, where you can see the sort of information included about other reviewers.