Editing, or the art of revising and perfecting your manuscript, is a large part of the writing process. In every writing class I took I had to write several drafts (I think the magic number was three) before I turned in a finished assignment. While editing may not be the fun part of writing, it is still one of the most crucial parts.
And editing doesn't just come in handy with writing. The world would be a much better place if everybody would give a quick look before sending a text, tweet, email, or status update.
1) Friends and family. Ask your friends and family who wouldn't mind helping you out. Who knows, you might find one or two who would love to do this. I know a lot of writers who have stated that their spouses are the first to read a draft before being seen by others. Drawbacks: These people are rarely professional and know what to look for. Also, they may not give honest feedback because they have a relationship they need to preserve with you and might be afraid to hurt your feelings. I've personally had little success with this and rarely get anything back beyond, "This looks fine. Yeah...."
2) Writing groups/critique partners. Look for a local writing group. If that isn't available then look for one online. Basically, you are exchanging services. You will send your writings critiqued in exchange for doing the same for those in your group. They aren't just great for editing purposes, they are also a fantastic way to network, make new friends, get writing ideas, and learn new writing techniques. Drawbacks: While you can get more honest feedback than you can with friends and family, they aren't always the best to be taking advise from. It helps if they write in the same, or at least similar, genre as yourself. I find that of the feedback I get back, I only take about a third of it to heart. Of course, that number changes based on the critiquer.
3) Professionals. There are people out there who perform editing for a living. You will get honest feedback and very professional results. Drawbacks: Because they do this for a living they aren't going to do it for you for free, so be prepared to spend a little bit of money for their time. They are also in very high demand, so don't be surprised if they don't get your work returned to you the next day. I've heard stories similar to, "Yeah, I'll look at it, but I'm booked the next three months."
As a side note, if you are looking for a professional editor then I have two I would suggest. One is a friend I made in my critique group who has recently taken editing up as a living. His name is Paul Yoder and I can't wait to throw some business of mine his direction. I haven't used his work yet, but I know him personally and consider him a friend and an honest individual.
The other is Tristi Pinkston, who has already edited some of my work and I've been nothing but impressed with her results. She is busy and was the inspiration behind the comment, "I'm book for the next three months." If you can wait three+ months I can personally vouch that she is WORTH IT. Well, it was three months the last time we chatted, it may have adjusted up or down since then.
4) Your publisher. When you publish your book, traditional publishers usually furnish an editor to clean up your story. A word of warning: don't wait until your book gets to the publisher. A publisher rarely pick up stories that haven't not gone through a few drafts already. A publisher won't take un-edited stories seriously and will only consider stories that are almost ready to be published and some minor fixing.