sometimes it just comes to u. ya know when your reading the word and u make sort of a melody out of it, u just make that melody into chords. sometimes it helps to listen to different songs and steal sort of the same tone. not always though. i've only been playing the guitar for 2 years and i've already made several songs. practice make perfect. try different chords or licks and see which one u like best then put the words with it.