@mirifique: Let’s take what I imagine to be the worst case scenario, where your friend takes the song, claims he wrote it, and sells it to a record company, which makes the Jonas Brothers sing it, and they sell millions of albums with that song on it.
The fact is that under US Copyright law, you had the copyright in that song from the very moment you wrote it down. If you never wrote it down, you had copyright in that song from the moment you recorded it as an MP3 or OGG or whatever. (Funny story: you can have two separate copyrights: one in the underlying composition, and a second in the particular recording.)
So, essentially, neither your friend nor the record company have a valid claim to that copyright. If either of them registered the copyright, a court would assume that they were the author, but you’re entitled to rebut that presumption at trial. It’s not ironclad — but you will have to make sure you can prove when you wrote the song. (Hint: registering your work is a really good way to prove when you wrote it, but not the only way.) You can sue anyone who made money by infringing on your copyright to your song for the profits you lost and/or the money they made.
Registration provides some really cool benefits but it’s entirely unnecessary in order to obtain a copyright. If your friend copies the song, they’ve infringed your right to make copies of the song. It’s as simple as that. If they remix it or just steal the words, they’ve infringed your right to make derivative works of the song.
If you’re unfamiliar with what specific protections copyright gives the author, check out the list. If any of your friends (or anyone on the planet) does one of these things, you have remedies under the copyright law.
Hope this helps!
Also: I’m not a lawyer, and this isn’t legal advice. If you have a question about copyright, you need to ask an actual lawyer.