This article is from the Copyright Law FAQ, by Terry Carroll with numerous contributions by others.
Good news. You already have. In the United States, as in most nations,
a work is copyrighted as soon as it is created:
Copyright protection subsists . . . in original works of
authorship fixed in any tangible medium of expression, now
known or later developed, from which they can be perceived,
reproduced, or otherwise communicated, either directly or with
the aid of a machine or device. 17 U.S.C. 102(a).
A work is "fixed" in a tangible medium of expression when its
embodiment in a copy or phonorecord, by or under the authority
of the author, is sufficiently permanent or stable to permit it
to be perceived, reproduced, or otherwise communicated for a
period of more than transitory duration. 17 U.S.C. 101.
What this means in simple terms is that as soon as you've created your
original work, it's copyrighted. Because of the "either directly or with
the aid of a machine or device" provision, it doesn't matter whether
you've printed it out, or if it's only on your hard drive or floppy disk.
You don't need any special formalities, such as registering the work with
the Copyright Office, or providing a copyright notice (notice stopped
being a requirement when the U.S. signed the Berne Convention and enacted
Berne Convention Implementation Act in 1988; see section 4.1 for more
That being said, you might want to register the work and provide a
copyright notice anyway. There are certain advantages to doing so (see
sections 2.5 and 2.7).