My first song was a result of thumbing through a chord book and coming across a Bm7 chord all fretted on the 7th fret. I accidentally added the open high E string, and it added a slight dissonant melody when picked after the chord rake. Liking this I searched on my bass E string for where I thought the next chord should be. I settled on G, and searched the chord forms of G with the root on E 3rd fret, and liked GMa7. Then adding some open string pulloffs on the B string for each chord, a progression was taking shape. I also found that the open string harmonics on the 7th were the same notes as the B chord, so that opened up another dynamic. You get the picture...writing a song is a combination of creativity, intelligence, inspiration, and of course just damn luck. You can't force it, but you have to keep at it.