The song is copyright protected as soon as it was made. To register the copyright (and obtain more protections) you will need to see a copyright attorney to properly file the work (and to advise you as to laws governing copyright licensing requirements for sampling of copyright materials).
Note: I assume your sampling is the common use of a recognizable portion of another song as a portion (often repeating) of a new song. This use is typically not a "fair use" of the sampled song and therefore without a license for the use of the sampled song, would amount to copyright infringement of the original (sampled) song.
With a license, your new work is a derivative or transformative work of the original song and is immediately copyright protected, but the ownershp of the new copyright is shared by the original copyright holder and the new artist (to the extent that the license agreement with the original artist provides). All sales, distribution, public performing, posting, copying of the new derivative work can only be done with the consent of the original and the new copyright holders.
Feel free to contact me or any one of the excellent attorneys here in AVVO. Many of us offer a free initial consultation where you can discuss your questions and concerns in confidence, which this public forum does not provide.
My disclaimer is simply that Avvo already has an adequate disclaimer.
Visit the website for the U.S. Copyright Office that I have produced below. A copyright registration for lyrics would use Form TX. The website features a lot of information and forms, including an online application form, on how to protect the different types of work in which you could claim copyright - performance, audio, video, lyrics, etc. You should peruse the various "circulars" or brochures that also are available on that website and explain copyright law, copyright registration practice and probably answers to many other questions that you may have.
Incidentally, you have a copyright right in any copyrightable work (such as original lyrics) as soon as you write them, perform them or otherwise put them into a tangible medium of expression. The copyright registration merely registers that right with the U.S. Government which is important for providing public notice of your right, enabling you to sue to protect that right in a federal court and authorizing you to recover statutorily defined damages amounts if a court finds that your copyright registered right was indeed infringed.
Alex Butterman is a trademark attorney with Staas & Halsey LLP (http://www.staasandhalsey.com), a Washington, D.C. IP boutique law firm. Alex is admitted to the bars of Washington, D.C., New York and New Jersey but, unless otherwise specified, the answer is intended to be general enough to apply to any U.S. state and based primarily upon his knowledge and experience with applicable federal laws. The opinions expressed are those of the author and do not necessarily reflect the views of his firm, Avvo or other attorneys. This answer is for general information purposes only and is not intended to be and should not be taken as legal advice. No attorney-client relationship or obligations are established herein, although consulting an attorney to discuss your specific situation is strongly recommended. This is especially true of trademark law because TM law is so fact-specific and full of esoteric nuances and exceptions that could easily result in a critical legal error without proper advice from experienced trademark counsel.
You already have copyright protection. A generally held misconception is that some special action is needed to "copyright" something. It once was, but that was eliminated years ago. International treaties to which the US adheres eliminate formalities for protection of copyrights. The US has a copyright office where you can "register" the copyright protection, and by so doing gain additional rights, as my colleagues have already explained, perhaps chief among them is that under 17 USC 411 http://www.law.cornell.edu/uscode/text/17/411 pre-registration or registration is a jurisdictional prerequisite to suit for enforcement of a copyright.
You are not clear how much you changed what you sampled. You may have changed enough to be considered to have "transformed" the song into a new and original composition. Or, if you did less than that, and still have essentially the same song, you likely have either a cover or a derivation and need a couple of licenses to monetize. Your creation is likely registrable either way.
All of this means you need an agent, a manager, or an attorney to guide you as you clearly do not know copyright law. You likely have not considered trademark law issues or band agreements, if there is a band, or business entity issues.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Once your song is "fixed in a form" its copyrighted. Registration of your copyright gives you more protections, as previous attorneys have noted, but I actually recommend that startup artists not seek total "all rights reserved" copyright protection for their works. Rather, I encourage folks to read up on Creative Commons and the types of licenses and copyright protections available through their system. Creativecommons.org is their website, and it is a fantastic resource for artists trying to get their music heard by media that traditionally cannot afford licensing rights for protected works (venues, restaurants, college or public radio, etc.). While it is always nice to have a royalty for every use, proliferation of content is what creates big stars these days.
As for the copyrightability of a song containing a sample, take a look at 17 USC 107. These are the factors used by courts in determining whether your sample is "fair use." In most circumstances I've seen in both producing rap records and representing rap artists, the best case you can make for fair use is that your song does not directly compete against the sampled artist's work in the marketplace and the sampled amount is insufficient so as to create confusion about the author of the work. Long form samples (i.e. "under pressure"/"ice ice baby") where a full 16 bars is sampled, or more, might be troublesome. But, a short form sample (i.e. "Paper Planes" / "Swagga Like Us") where a sample is short, and often repeated several times throughout a song may be okay. Producer rule of thumb: if you could've sampled it on an MPC 60, you probably could win an infringement suit. If you need Pro Tools and time stretch to extract and produce your sample, you're doing it wrong.
Have an attorney on your team. It makes these judgment calls easier to make.
Answers to questions does not create an attorney/client relationship. I only am your attorney if I have entered into a written contract, signed by me, wherein I expressly assent to be your attorney. Nothing I post should be construed as legal advice to be acted upon, it is merely a legal opinion.