I am a singer/songwriter (though I haven't posted any on youtube yet), and I find the easiest way is to tell a story. Write it down, then turn it into a poem. From there, you take the music, put it with the lyrics, and put your feeling into it. A song is just music and words without feeling. Whether anger, sadness, happiness, whatever, put feeling into it. Music moves people emotionally one way or another. Move people and you will have a great song.
If you need help, let me know what you are trying to do. I can probably help you come up with lyrics.