Typically, the first draft of a novel is the one that will be riddled with errors, and that is fine. At this early stage, you are just putting together the skeleton of your story, though the more careful you are, the less difficulty you will have later on. For now, enjoy the creative process and the thrill of creating something unique.
The work does not finish when the story ends. Once you have written the first draft, go back through and check your spelling, structure, plot, typing errors and so on, and make any changes. You are almost certain to find something you do not like, so this is your chance to make amendments, also known as the second draft.
Even after the second draft, I would still recommend going through your work one more time to look at the novel in more detail. Make sure your punctuation is accurate and that the novel flows well. It is also worthwhile getting your formatting sorted here as well, especially for those authors opting for the self-publishing route.
Once you feel you have completely exhausted yourself and made sure you have eliminated the errors, this is where a copy editor and/or proofreader comes into play. Because they are completely neutral and offer a fresh pair of eyes, they are more likely to pick up on any errors you might have missed. In addition, they can also approach your novel from the viewpoint of a reader; they can let you know what the reading public might think of your work, as well as providing valuable critique before you send your manuscript to the publishers.
What is important to remember is that only you as the author can know for sure when your work is ready. Trust your instincts; if you know that you simply cannot do any more - and if you've used a copy editor - then you can take the plunge. Make sure you're happy with your front cover and blurb (again, asking for other people's input can be a huge benefit here) and take the leap of faith.
If you would like my help in getting your work to this 'ready to read' stage, visit the Services and Rates page, or send me an email.