You need to get in touch with people, on forums, soundcloud, facebook, twitter. And find someone that can add something to your song (or you can add something to their song). Talk about the process, how you imagine the song will sound like, and try to be in an unison with that person. From there, you can choose any way you want to woth with him/her: exchanging wavs, project files.
It's not that hard once you two have decided where you want to go with the production of the song.