Keyword research will be the foundation of your book marketing, so it's important to spend some time on it and do it well. Even if you've written a book without a market in mind, you still need to identify the best way to position your book, by finding your closest competitors, keywords and categories, so you can give your book its best chance at success.
You also need to know exactly which keywords your target readers are actively searching for, so you can use those keyword phrases in your description, in blog posts, and in your advertising or marketing.
The video above is a basic introduction to keyword research. The video below is a 45-minute overview of the tools I use, how I do my research, and about a dozen other crucial processes that will bear big results later (sorry for the length; I actually recorded a 15-minute version I thought was too unfocused, then when I refilmed it it got even longer). You can skip it if you don't have time to watch it right now, though it's probably one of the most important videos in this course, since I mention a lot of the pre-marketing things I do that make it easy for my books to stay visible and keep selling long term.
IT'S NOT TOO LATE
It would be more profitable to do all this research before you start writing, to figure out what kinds of books readers are looking for, and then write one of those - some authors do that. If you're just writing crappy books quickly to cash in on hot trends, no it probably won't work. That said, publishing trends don't actually change very much.
Vampires are still 10x more popular than mermaids, and that has been true for the past 20 years; even though 2017 should be a big year for mermaids, with several live-action films and a bunch of new YA titles, they still won't outperform vampires - there's a difference between what the publishing industry guesses might be the next new thing, and what readers actually want to read.
But don't assume the popularity of trends doesn't matter. Urban fantasy tends to sell much better than scifi-dystopia. Contemporary romance is a much harder sell (there are also very large genres, which could be profitable if you can outperform everyone else, but the markets may be more competitive.)
You can still write what you want to write, but your books will be more successful if you align them with what readers are searching for and always try to rank on the first page (top 20 results) in category and keyword searches. Don't think about this as selling out or changing what you write. It's still your story; you're just positioning it for maximum visibility by aligning it with keywords readers are actively searching for.
Spend an hour doing research and write down...
10 books and authors that are similar to yours (and your target readers are searching for).
e.g. Hunger Games, Twilight...
10 "long-tail" keyword phrases or genre categories that fit your book
e.g. technological thriller, historical regency romance, space opera adventure...
10 websites, lists or blogs that show up on the first page of Google when you search for those keywords. This can/should include any Goodreads lists or categories... but try to focus on just sites where you might be able to contact them and get your book added.Down below are some resources you can use in your research. I forgot to mention Neil Patel's tool in my video, but it works pretty great and is free.
I would try KWfinder first:
It will show you recommended keywords, the average cost per click for paid advertising, and will also show you the top websites that rank for that keyword (saves you some time).