It depends on the country you're in and which society you want to use. In the U.S. it's ASCAP and BMI, in Canada it's SOCAN. There's a ton of other societies for other countries.
There's certain criteria you have to pass before you can join. Like you need to already have a song playing on the radio or tv.
Once you've got a song being used somewhere, the producer will send you a sheet with all the info on it like what show it is, who the producer is etc... and what song is being used. You send the sheet into ASCAP or whoever, with your application.
Once you pass the basic rules, you're joined.
Then you get an account so you can log on and list all your songs online.
If any of your songs are used anywhere, notify the society and they'll collect royalties for you. They also collect royalties automatically from major broadcasters and radio stations.

I'm with SOCAN in Canada and ASCAP in the U.S. So for instance, if you were to register with ASCAP or BMI, I could automatically link up your name to this song so that the royalties would be split before they're sent out.
If you can't join a society yet, I'm registered as a publisher so I can collect royalties for you and send them out. If a time comes in the future when this song is actually going to be used and there's a possibility of it making some cash, we'd have to write up a contract stating what your percentage is and that I'm representing you etc.. until you could join the society yourself.