In the U.S., you can register copyright on any creative work, including writing, art, etc., once it has been set down in tangible form. In other words, you can't register just the idea, you have to actually create the work before you can register your copyright. It used to be that you had to fill out some forms, and send them, along with a copy of your work and, of course, a check, to the copyright office.
Now, finally, the U.S. Copyright Office has come up with a way to register copyrights electronically. You can fill out the form, pay by credit card, and even upload the work in almost any of the commonly used formats. There are still some formats and works that require sending a hardcopy deposit of the work, but there's a vast amount that can be processed completely electronically. You even save $10 on the fee.
There's another alternative that lets you fill out the copyright form on-line and then print it. You then mail the printed version along with your work. The printed version will include bar code data of the fields you enter, so it can be processed automatically once it's received by the office.
For more info on all of these options, check out:
Now you have no excuse for not registering!