what normally happens is the band gets together and records a songwriting session. you can just a use a simple tape recorder.
whoever is in the room contibuting to the songwriting generally gets an equal share. Best to only have band members in the room or if you need help you can bring in someone from outside.
You can also agree to percentages or you can agree on paper who wrote what. Like maybe someone just wrote lyrics and the rest of the band wrote the music.
you don't need to be able to write sheet music you only need to record it for it to be copyrighted. Then I would highly recommend registering your song with
www.copyright.gov
That way you have a record of registery that will hold up in court. Mailing a CD and lyrics to yourself (poor mans copyright) does not hold up in court.
You will need to produce the song on CD and also type the lyrics and mail them off with payment and copyright form to the copyright office in Washington...
www.copyright.gov
hope this helps. good luck and have fun writing.