Don't do it.
The first draft of anything you write needs to be your little secret. Hug it to your breast, murmur sweet nothings to its pages, but don't let anybody read it.
Why? Because it is filled with mistakes. There are errors in grammar, punctuation, spelling, word choice, internal logic, voice, awkward constructions, typos. There are whole scenes you have left out, dramatic high points you have missed. There is a relationship between your characters that you have not fully developed. You may even have used impact as a verb! (AGH!)
If you think none of those things appear in your book, you are wrong. If you think nobody will pay attention to any of those minor blips, you are wrong. If you think your glorious prose will shine through, like the sun breaking through clouds of dangling participles, you are dead wrong. Those errors will mar your work.
At this point, I may have sufficiently scared you into investigating editors. Don't do it yet. First, give your book a once over. Maybe a twice over. Or a three times over. But take a good crack at it before you send it off to be edited. (You will save yourself some money if the editor doesn't have to fix quite as many mistakes.) Follow these steps:
1) Put your manuscript away for a few months. It will grate on you to simply park your project for awhile, but I can guarantee you need a break from it in order to see it with fresh eyes. You can use the break to write something else. Or you can read. (As you read, ideas about how to make your manuscript better may pop into your head. Either take notes, or make a few changes.)
2) When you pick up your manuscript again, read it aloud. Reading aloud slows processing speed, allowing you to spot errors more easily. If you stumble while you are reading, revise that sentence or phrase. You've composed an awkward or ambiguous construction that readers will find difficult to parse.
3) Open your book to a random page to edit. Writers get "manuscript blindness" when they read through their work from first to last page. It's a natural consequence of looking at something too many times combined with the inner knowledge of what comes next. To find errors you need to surprise yourself.
4) Read your first page. Pretend that your book will be judged on that page, and that page alone. (Often, readers don't even make it past the first paragraph. I know I don't.) Polish that page until you can see your face in it.
5) Repeat step 4 for every page in your book.
Now you can show your manuscript to trusted friends or beta readers. After you have gotten enough feedback to hone your book even further, you can begin searching for an editor. At this point, you will have a much better idea of what kind of editor you want (line editor, proofreader, or, if you are still stuck on structural components, a developmental editor). This article will help you find the editor who is right for you: Costs for Editing a Self-Published Book
When, at long last, you send your book to an editor, she, or he, will find errors, because there are always errors. But you will have had the chance to take care of plot holes, character motivation, missing scenes, and all those things that only you, the author, can do.