(Note: Don't panic when you read the following. It's scary, but you are more than capable of handling it. If you've come this far, chances are, you're a good writer.)
You've decided it's time for a professional edit. Before you begin, there are some questions you need to ask yourself.
How much are you willing to change? Are you willing to receive suggestions for rewriting scenes, such as "I have trouble believing this character is this age; can you make them 35 instead? Here's why." What about suggestions for scene inserts: "I'd like to see a scene of this character arguing with her brother about how he keeps cramping her dating style, instead of her reflecting on it."
There will be suggestions on scene cuts: "Start this chapter here; cut all this." "Too much reflection. Start the chapter 2 weeks ago, show this scene, cut the reflection on the scene, skip time to current day." Your favorite scene: "Head-hopping: you've never used this character's point of view before. And they're not important to the story. Cut this scene or rewrite from someone else's point of view."
What about character development issues? "This character seems flat" or "This action doesn't make sense for this character. Either explain why he does this particular OOC thing with a scene from his POV or make someone else do it." "Alpha males don't cry over smacking mosquitoes. Symbolic or not, it seems melodramatic and doesn't make sense for him."
Even the best writers get back pages and pages of things to improve during their first few books (and most for the rest of their careers).
I warn you now that your first beta-edit from someone who writes or edits professionally is always, always hard, because these are the edits that are focused on making the story better and more in line with industry standards of writing--which are much stricter than they were 20-30 years ago, meaning the authors we grew up reading get away with stuff we can't. Even if you've got thick skin, it'll make you want to cry. Why? It's just overwhelming to look at a story and see how much someone else thinks should be fixed--although after you start fixing, you realize it's not as overwhelming as you thought.
Good edits push you to move past "good enough" or "at least as good as that other author I've read" into "excellent." That means the stuff you see other authors getting away with, your editor should call you on and tell you to fix it. Whether you do or not is your choice (or not, depending on how and where you publish, and what your contract may be).
One of the best things you can do as a professional (and if you're publishing and earning money from your books, that's what you are) is to get feedback from someone who doesn't know you particularly well, and who won't try to protect your feelings. That's also the danger of asking friends for beta reads, because close friends aren't willing to point out the things that hurt (and it's always the things that need the most improvement that hurt the worst!) This is why a professional beta read is always a shock, and always painful, even when it's done tactfully. And after giving yourself a couple of days of frustration, tears, and well-earned self-pity, you'll start making the changes--and be hooked.
Seriously. Once you get a real edit, you'll never want to go back to the "this is so great!" again. You won't believe me until you feel it yourself. But as a writer, it's the honest feedback that makes you improve. And once you see what you're capable of, you'll never, ever accept less. Which is why writers treasure professional, tear-inducing edits so much (note: strict can be good, but never "abusive"--don't go with an editor that insults you. Ever. You are a good writer, and you do have what it takes. Someone tells you differently, hit upside the head with a frying pan. I guarantee you, there are pro authors whose first drafts are worse).
I know this all sounds scary. That's why I giving you this warning. Everyone goes through the same thing. I promise. And I also promise that it's worth it.
If grammar or punctuation is an issue, check out OWL at Purdue. Read the whole thing. Seriously. Read it and take notes (study secret #1 for online classes: taking notes by hand improves understanding and memory retention). Your punctuation will improve and you'll know how to avoid the little errors. It's worth taking the time to do.
Grammar Girl is also good for specific questions, especially for commonly confused terms or punctuation issues.
You are a good writer. But everyone can be better, and that's the point of editing. Never stop reaching for the next level up.
Published authors: What was your first professional feedback experience like? How do you feel about professional edits today?