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.
Mechanical royalties are the main source of income for publishers.
The current statutory rate for mechanical royalties is 7.1 cents per song per record for recordings of up to 5 minutes in length. For recordings over 5 minutes in length, the rate is 1.35 cents per minute, or portion thereof. The statutory mechanical rate is subject to change, and should be checked before any project. So far, it has only gone up, so it is beneficial to the song's copyright holder to negotiate a provision that the rate will be calculated as late as possible, i.e., the date of a commercial release, rather than the date of a contract. Ideally, the record company and the song's publisher work out the details of the mechanical license before the song is recorded and placed on a compact disc or other mechanical reproduction.