Have you spent some time away from the manuscript?
The time you need varies, but always spend at least a week away from your project. Read something totally unrelated or write something else. Give yourself space to gain perspective. Personally, I like a month's worth of distance. It's great that you're excited and want to put this project right into an editor's hands, but a first draft is never ready for that level of scrutiny.
Have you read through the manuscript several times?
After you've gotten some distance from the project, it's time to read it. You should always be your first editor but not your last. It's a tricky thing, self-editing. Sometimes it can go too far. As a safety net, when I delete sections or heavily rewrite them, I save the originals into a scraps document just in case I find them necessary later. At the very least, clean up typos and grammar mistakes before having someone else read your manuscript. You don't want them distracted by those small annoyances when there's a story to be told.
Has someone other than you read it and critiqued it?
I mean someone unpaid: a friend or family member who's honest, beta-readers (you can find them on Goodreads!), members of your writers' group, etc. What you want is a reader's perspective, especially one who is familiar with your writing. This step is so important because it helps prepare you for criticism. It's often gentler and easier to hear coming from people who love you as well as your work. They might notice some of your more frequent missteps (see my post on Breadcrumbs for more about habitual flaws). Getting the obvious issues out of the way ensures that your editor won't be telling you something you could have figured out for yourself.
Are you ready to put in the work?
When you get your manuscript back from the editor, there will be a lot of work to do. You will likely go through several rounds of editing. When you decide to hire an editor for your manuscript, you're making a commitment to that project. If you've fallen out of love with the story you wanted to tell, it might be better to move onto a new project. Although you might feel you're not getting the manuscript right yet, you should still see a little shine left on your original idea.
Now, you're ready to hire an editor, so let's get started!