Choose an emotional theme for your song, like love or sadness. Write some simple lyric lines with rhyming ends and separate them into verses and a repeating chorus.
Choose a simple key to sing that matches your voice range. Most popular songs have a short vocal range and usually stick close to middle C or the few notes above or below it. The key of D major is a good first choice.
Play the 1, 4 and 5 chords in your key. Most popular songs use the chord progression of 1, 4 and 5. These numbers refer to the steps in the key you have chosen. The first note in the scale is 1, the second note is 2, and so on until you count to 8. In D major, these would be a D major chord, a G major chord and an A major chord.
Think of a simple melody that will fit with the notes you are using. Play around with a sequence of chords that you like the sound of. Keep it simple and let the notes repeat when you can. The easier the melody is to sing, the easier it will be to remember. Use this first melody as your chorus. The chorus of your song repeats between verses and refers to the emotional theme or the moral of the story you have chosen to tell.
Choose a melody for your verses. Remember that the words will change with each verse, so the tune should again stay fairly simple, to accommodate the words.
Put it all together. If you don't like the result, return to Step 3 and rearrange your chord progression to find a better fit. Keep trying until you like the result.