Here are a few random thoughts to add to the excellent advice given above.

1. Listen to everybody's advice, then do what works for you. One method may work for one person, and not at all for another. Similarly, one method may work really well for one song, and not at all for the next 10.

2. The very first song I ever wrote (when I was 6 years old) was a parody. I've written many parodies since then, as well as other stuff. I still maintain (pace Mary Garvey, who hates parodies!) that writing a parody is a great way to practice putting lyrics to tunes. The structure is already laid out for you, and you can borrow some of the phrases.

3. I'd recommend a book called Writing Down the Bones by Natalie Goldberg. It's about writing, not just songwriting, and I think it's wonderful. The biggest lesson: turn off your editor while you're writing.

4. Another of Natalie's lessons that is very much applicable to songwriting is: use original detail in your writing. The songs I find most moving are often the ones that paint a small but detailed picture, rather than the ones that preach a generalized sermon. Michael Smith's "The Dutchman", Stan Rogers' "Lies", are two that come to mind.

Most of all, have fun!   

Aloha,
Mark