Is it fiction or non-fiction?
If it's fiction, keep writing, and rewriting. Join a writing group germane to the subject of the book. i.e., mystery, children's, SciFi, etc. It's better to have those within your genre as peer review. Have them give you a fresh eye to your manuscript, first few chapters, etc.
Then when the manuscript is to your liking, you can send it to the Library of Congress for copyright registration. They recently upped the fee to $45 =O
You do not have to do the above, tho: It is already copyright once you put pen to paper.
However, please note that you cannot copyright an "idea." You can only copyright works. Ideas must be trademarked and its a lengthy expensive process and not necessary for books in early stages (if at all). i.e., only something with wide mass audience appeal such as Tolkien would bother with copyrighting names like Gandalf, the word Hobbit, etc.
If it's non-fiction, take a course in how to write a book proposal. A book proposal can be shopped to agents and publishers without being totally complete. However, you must do a bit of work and have justification for why YOU should write this nonfiction book. Peruse the web and you will find much resourcewise as to how to do this.
