most styles of music rely on blatant plagiarism, some admit this, some don't...
This is true but that's not how I'd phrase it -- this is how music has existed since the beginning of time, and nobody ever thought of anything in terms of 'originality' until it started to become a marketable commodity, at which point you had to start pretending, as with any other commercial product, that you didn't rip off the idea from someone else.
I forgot the statistic, but something like 75% of every word and note Bob Dylan ever recorded is unoriginal, and he didn't pretend otherwise because he came out of the folk tradition, not the pop tradition.
@ratscratch I don't know if this is an answer to your question, but here's how I write songs: Start with the lyrics, think about what their mood is, then choose 3 or 4 chords that suit that mood. Then I strum and sing along with the same notes as the chords I'm playing. When I've gone through the song enough times, I start to think of ways to embellish the vocals.
Probably not the best way to do it; I don't even begin to understand musical theory.