A song's copyright holder owns several important rights, including the right to copy the song, distribute copies of the song, prepare derivative works from the song and perform the song publicly. In the beginning you, as the composer of a song, are the song's copyright holder. In order to get the song to generate revenue for you, you may find it necessary to affiliate with a music publisher.
Music publishers come in all shapes and sizes.
You want to affiliate with a publisher that will be able to find users for your song, issue licenses to users, collect the revenue, and then share the money with you. You may transfer all or a portion of the copyright to your publisher, which will, in turn, affect the way revenues are split. Depending on your bargaining power, the publisher may give you an advance against future revenues. As background, you need to understand exactly what kind of money a song generates for its copyright holder. The way you get the money, and how much of it you get, depends on its source.