You're pretty much asking for a couple of years lesson in a thread. It's a bit unrealistic... :-/
I usually start with the drum loop, or feeling, or whatever (since I'm doing the production myself), and also a pattern on the strings, or the guitar, or the piano, etc... Then comes the harmony and at the same time I get a feeling on the lyrics (which most of the times are my own) and work out the melody according to the harmony. I'm a harmonic man, not a melodic one.
Here's a song I did somewhat recently...http://www.youtube.com/watch?v=ynbTYEv1egE
The strings came first, and then the voice melody, along with the lyrics. Everything else came later on...