Write down the song details. The title and credits must be correct when registering the song, so it pays to write them down. If you have co-written the song, have the other writer check the details for accuracy.
Register with a royalty collection agency, for example the American Society of Composers, Publishers and Authors (ASCAP) or Broadcast Music Inc. (BMI). These are the two main agencies that collect publishing royalties on behalf of songwriters. For ASCAP visit www.Ascap.com//join and click "Writer." Have your social security number, song details including title and writer credits and a debit or credit card ready. As of June 2011, ASCAP membership incurs a one-off fee of $35. For BMI, visit www.BMI.com/join click "Writer/Composer." BMI membership is free, but they take a cut of your royalty. The amount depends on the type of royalty. It's not necessary to register as "Publisher" because you already own the rights to the song and can therefore administer those rights as you see fit.
Fill out the online membership form for your preferred agency. Both require you to enter the name, age and address of all writers. The address you enter on the membership form is where your royalty check is mailed to. If you don't plan on staying at your current address for long, don't use it. You can update your details later, but to avoid checks going to the wrong place, use an address that you know is long-term.
Register for an online music sharing site, such as Bandcamp or Soundcloud (See References 2 and 3). Both have quick, simple registration procedures and both are free. There is no reason why you can't register for both.
Upload your song to your chosen music sharing site. For Bandcamp, log in and click "Upload." Browse for the MP3 of the song and click it. Once it's uploaded, copy the link and paste it into a document or email for safe-keeping. For Soundcloud, log in and click "Upload and Share" and click "Choose Files." Once uploaded, copy the link. This provides an online home for the song so you can show it to potential licensees conveniently.