There is no easy way. Learning to do it properly will take a fair bit of time and effort. Perhaps you could pay someone to do it. Alternately, you could write down the lyrics, and play the melody on a keyboard, and have some software capture the midi data being sent in and you could have it scored that way.
To suit your specific purpose, though, you don't have to do any of these. Simply make a recording of the song - as rough as you want - as long as the lyrics and melody are there. Put the recording and a lyric sheet into an envelope and mail it to yourself. Don't open it except under observation from a judge, lawyer, notary, etc. This won't officially register it, but it WILL prove you OWNED the song (or at least a copy of it) on the date that the post office will stamp on the envelope. If nobody else can prove ownership prior to that date, then it is assumed that you must be the creator. This isn't 100% idiot-proof, but it's pretty good.
Chris
