Ah, that's the hard part!
First make sure you have a good book, a great title, and the best cover you can get. People do judge a book by its cover, so make sure they think yours is going to be great.
You need to be a messenger for your book. You need to talk to people, present your work by reading selections at open mic nights and arts festivals and such. Get on line at places like Goodreads and interact with people, letting them know about your book.
If you have a special interest in the book - domestic abuse, fantasy adventure, fairy tales, find communities talking or writing about that online and join. Don't just be a ***, become a part of the community and then tell about your work. Also, mention it to friends, family, and so on.
Have cards printed with the name of your book, a picture of the cover, and details about how the person can find the book online.(I'm not plugging them but Vistatprint is a cheap and easy way to go.) This makes it easy for people to find you. BE SPECIFIC.
Check and make sure that searching for your name and title actually takes you to the book. You might be surprised. I had to work a bit to come up with search terms that take a person right to all of my published books. The Amazon Search kept throwing up a translation of this Russian book with the same title instead. Don't let this kind of thing happen to you. If necessary change your title to make it more searchable.
Give cards out at readings, to family members, to anyone you talk to. If you are chatting to someone in line at the grocery store, or whatever, say, "Oh, yes. I've written this book. Here's the information on it. You might want to check it out." Be brief, confident and positive.
Make it easy to find and buy your book. Also, put your book in Kindle Select, and give copies away for free as often as possible. Believe it or not, more people will then buy your book. (No idea why, but it works)