Think of a title. A great title is a good place to start. It serves as your hook for the song or your chorus. Don't discriminate when you begin to write. Your song can be about substantive things or inconsequential subjects. Think of action words and short catchy phrases. Look at newspapers for headlines you can use. Find interesting, attention-grabbing phrases in books and magazines.
Contemplate your title and answer the questions it asks. For example, if your title is "I Bought it at the Store." Ask yourself what you bought, why you bought it and why you needed it. Write the rest of the song as you discover the answers to the title's questions. Consider adding random details to give your song an interesting twist.
Find the structure. Begin with a ghost melody, a melody from a familiar song, to discover how the lyrics sound to music. This helps define the structure of the song. Typical songs use the structure: verse, chorus, verse, chorus, bridge, chorus. Tell your song's story through that structure.
Play with the melody. If you play an instrument, use it to create your song's melody. Choose random chords at the piano or guitar to begin your song. Listen to the chords as you think about your lyrics. Find the chords that fit the mood of your song. Use the pattern of the rhythm of your lyrics to find the natural rhythm of your song. Make small adjustments to the rhythm and melody until you find the music that fits your song.