You don't need to - the very act of writing/creating it gives you automatic copyright, but you would need to be able to prove the date of its creation in court if someone nicked your book. If you created it on a computer, then it will already be dated when you save it and even if someone wanted to change the dates, the information would remain encrypted on your hard drive. If you're feeling a bit paranoid about it, you could post a paper copy to yourself and leave the envelope sealed when it's delivered, but it's not necessary.
When it comes to putting copyright information in the book itself, your eventual publisher will take care of all that for you.