You may have an idea or concept of what the song's going to be about first, but the music always comes first. Lyrics without a melody in mind are poetry. Can you convert a poem into a song? Sure, but it's not common in any genre other than rap. Most lyricists write with a melody in mind. Even if the music is in their head, they usually have an idea of how the basic melody is going to go. Now, in the production process more instrumentation may be added or use of dynamics may be employed to enhance the lyrics, but, again, the basic melody of the song is almost always in place.