First, a little copyright law - If compose/write a song alone, you automatically own the copyright at 100%. There is no need to register your copyright with the U.S copyright office. If you have tangible proof that you wrote the song (a recording, sheet music, a DAW session, etc.), you can easily prove that you are the original author. You can find more on that here.
So, if you wrote the song alone, you own 100% of the copyright. What if you work with a cowriter, a producer, or a larger team? At that point, everything is up for grabs. All percentages are negotiable. Often cowriters will split publishing 50%-50%, or 33%-33%-33%. Sometimes a producer or audio engineer takes 10%-50% as part of their deal for working with you. It all depends on what is negotiated.
In any case, once you and your co-writers have decided on splits, it is important that you get an agreement in writing. The Songtrust Team has prepared a split sheet template for you to use.
Please view the below diagram for some examples of splits. Once you've decided on splits, you can register your songs and begin collecting!