Decide whether your theme song should contain lyrics or merely be instrumental.
Unless you choose to compose an instrumental piece, write lyrics that give an overview of your subject. For instance, if you are writing a song for a video production about friends, your song should say something about friendship.
Choose a melody and rhythm that reflect the nature of the subject. For example, if you are writing a song for a commercial about a serious topic such as preventing abuse, you should use music that sounds appropriately solemn. Minor chords and slow rhythms often reflect seriousness. If you are writing a song for a band who specializes in playing upbeat party music, write a theme that also is upbeat and lively. Major chord and faster rhythms usually express energy and positivity.
Create a sound for your song that is original. Don't try to copy another piece of music. A theme song should be associated solely with a particular subject. Using a piece of music that sounds like some other composition can lessen how well your theme is recognized and paired with its subject.
Keep it simple. Simple melody lines, rhythms and lyrics make a song more likely to stick in the minds of listeners.