Reviews help new readers trust an unknown author. You cannot buy honest reviews, and you should not try, because retailers may remove them and penalize accounts. Instead, build a group of advance readers. An advance reader copy, or ARC, is a free copy given before launch in exchange for an honest opinion.
Recruit readers from your email list, your social communities, fellow authors in your genre and book bloggers. Send the file early, give clear instructions about when and where to post and thank everyone, whether or not the review is glowing. Do not demand positive reviews. Retail platforms have their own rules about reviews from friends and family, so read them first and never trade reviews for payment.
Keep a simple spreadsheet of who agreed, who received a copy and who posted. After launch, include a short note at the back of the book inviting readers to leave a review, since many readers are happy to help if asked politely. Over time, a reusable reader list saves effort on every future book.
Remember that advance readers are volunteers with busy lives. A gentle reminder one week before launch and another on launch day is plenty. If someone cannot finish in time, thank them anyway, since they may help with the next book.
Reviews do not only come from readers. Editorial reviews, awards and endorsements from respected people in your field can add credibility, particularly for nonfiction. Ask early, because busy people need time, and make it easy by sending the book, a short summary and a clear deadline. Never pretend that endorsements exist, and use quotes only with permission.
If a review is negative, do not argue. Every book gets some, and a mix of ratings tends to look more natural to buyers than a perfect run. Learn from useful criticism, ignore the rest and keep writing. Patience is part of the job, since reviews usually arrive slowly.