Tough question....and great answers have already been given. My 2 pennies are that it's obviously different for everyone where a song comes from. For me, I was writing songs before I ever learned how to play a single not on the guitar. I wrote poetry (still do), since I was 7 years old, so when I learned how to play I simply looked through the tons of material I had, changed a bit here and there and created songs. It just takes one thought to spark a novel, poem, song, or whatever creative experience you are trying to achieve. It's hard to give advice on how to be able to write a song....most people just do it the best they can. The key is never trust the first draft....always revisit and rewrite. The gist at first is to just get your idea on paper the best you can...don't worry about language or diction, that comes later. Let the piece stew with time for a bit....then revisit it and change stuff around. The great songs aren't products of one spontaneous songwriting orgasm, it's a process of constant revision.