Make a note of any phrases or ideas that stick out to you. Coming up with a song title first is a good way to get started if you aren't sure of a topic for your song. Song titles can come from anywhere, including films, newspapers, books and conversations. Any phrase that catches your attention could be a good starting point for a song. Write down possible song titles throughout your day-to-day life.
Think of any ideas, phrases or words that are related to your prospective song title. If you have an existing idea for a song title, or a topic you wish to write a song about, think about what you want to include. Try to answer any questions that present themselves when you look at your title. For example, starting with the song title "The Trees Are Burning," would instantly make you wonder why the trees are burning. The song could then evolve to be about the way humans abuse nature for their own needs, or alternatively it could be about a horrific forest fire. Choose a direction and work with it by writing down associated words and phrases that come to mind.
Play a basic skeleton song structure if you play an instrument. Strumming a few chords on a guitar or playing a basic backing on a piano can help you come up with a vocal melody for your song. Play a basic backing, and then sing nonsense words or "la la" over it in a melody that suits the tune. Listen to some songs by your musical influences if you need help coming up with a vocal melody. Think about how they choose to sing over their backing. As you gain familiarity with your vocal melody, add in words relating to your prospective title or subject. Think about parts of the song suitable for different sounds and words. Work out the general tune of the vocal so you can fit the syllables in your lyrics into the music.
Write the full lyrics. Expand your idea as much as possible, and incorporate any key phrases or words where possible. Add in imagery and metaphor to better convey your meaning. Keep song structure in mind when writing your lyrics. Use a basic song structure as a guideline if you aren't sure, such as verse, chorus, verse, chorus, bridge and then chorus. Write down a first draft of your lyrics, trying to hold back any self-critique. Try to get the words out without judging yourself.
Revise the lyrics after one day. Unless you are an exceptional lyric writer, revisions will be necessary. Go over what you wrote previously and see if any words can be changed to better convey the meaning, and alter any uncomfortable-sounding or unnecessary passages. Improve upon your basic idea, whittling away any awkward sections, until you have a song you are proud of.