Finishing your book and writing "The End" at the end of your manuscript is a great accomplishment, and you might feel like sitting back and basking in that good feeling. (maybe just a little) Indulge yourself for a day or so, then get back to work. A finished rough draft is only the first of many versions you'll write of this book, and you'll need some help along the way.
You might think of an editor as someone who slaps your manuscript into shape for publication, but that's only true in the broadest sense. There are different kinds of editors, each of whom does a different job, and you might need two or more of them for your book.
While beta readers aren't normally considered editors, they're definitely the first step in the editing process. Once you've gone over your book a few times to get it into readable condition, you'll need a few trusted friends or relatives to read the entire manuscript. Beta readers stand in for your readers; they're supposed to read the book just as a casual fan would, and make notes about problems they find. Then there are expert readers for non-fiction that can give you comfort that your information is up to snuff. Compare the opinions of all your beta readers. If everyone says that a plot point doesn't work or certain data is questionable, go back and review the issue and then fix it.
Proofreaders or Copy Editors
Proofreaders, or copy editors, are meant to be the most nitpicky of editors. They go through your manuscript and pick out every spelling error, misplaced comma, incorrect emdash, and missing capital letter you're guilty of. A proofreader is in charge of making sure your spelling and grammar are as perfect as can be, to allow for other editing to take place. They have nothing to do with the story itself, only the words and punctuation that make it up.
If proofreaders polish the words themselves, line editors make sure their meanings are correct. This type of editor looks through your story itself and looks for continuity problems, plot holes, factual errors, characters who simply disappear, and other instances of problems with the story itself. Line editors are the most intense kind of editor most writers will work with. Whether you're looking for an agent or planning to publish the book yourself, a line editor is the least you can do before approving your book for publication.
A substantive editor can get so involved with your manuscript that it may seem as if she's become another writer without credit. Substantive editors look at the story as a whole and decide where it hangs together and what parts of it need to be redone. If you've got a Stephen King-length manuscript, the odds are good that you've got parts in there that should be condensed or eliminated. Books with serious flaws but great hearts can benefit from substantive editors, but there have to be great bones to the story for it to be worth the effort.
Remember the old saying, "The more bookbrains the merrier" (AG, 2014). Live within your means and look for resources that can add that extrabit of insight that can make a difference in your finished work!
Keep the Faith and may the Force be with you!