When you sign up your book on the Kindle KDP site, they ask a lot of questions. Be sure you answer every one, particularly the ones about tags and categories.
Tags are the labels that tell readers what your book is about. Kindle will let you choose 6 or 7 of these. You want to choose words that readers are most likely to enter in a "search" box. Make them general to capture as many searchers as possible. Check various SEO sites for suggestions on the most popular searches. But be sure to be honest. Readers won't be tricked more than once. If your tags include "hot sex" you'd better have an x-rated book to go along with it.
[SEO = Search Engine Optimization]
Categories, on the other hand, are the labels that determine what best-selling and top-rated lists you qualify for. You only get two of these, so make them as narrow and specific as possible. Use "Fiction" as your category and you'll have to compete against millions of other books for a place in the ratings. Limit it to "Fiction - > Cold War -> Russian Steppes -> Romance" and you'll have a list all your own, where you can be #1 if you sell a single book. (That's an over simplification, of course, but fitting your book into a genre that is not over-populated can make a huge difference in your rankings.)
Rankings, you say? Can I really influence those? Yes, you can. We'll talk more about that topic next week.