Watch the Create an Intro online guitar lesson by Ravi from 1-2-3 Songwriting
Perhaps by this point it is obvious how to get into the song. If so, then the intro has probably written itself. But try a few different things in order to present your song. Maybe you want to begin with the chords of the verse. Or you want to come in right away with the vocal. Perhaps you hear a drum fill or beat setting up the song. Why not an ethereal keyboard sound fading in from nowhere? There are a lot of possibilities, but since it is just you and the guitar, imagine how you would perform this song solo-acoustic and experiment with ways to set up the song. You can always change it later, but for now, experiment with some different chords and progressions extracted from later in the song, and try to use that up front. Check!