One way to start working on a song is to think about its potential structure. Will it have a simple progression that repeats throughout, with the melody changing over it? Will it have a verse, chorus and bridge, each with a different set of chords? Play some classic sounding progressions, like a I IV V blues (E A B in first position on guitar), or a '50s and '60s style doo-wop I vi IV V (C Am F G). Start simple at first. Think about what chords sound good as a transition from progression to progression. A song with well-linked verses and choruses will flow better.
Try thinking of a general lyrical theme. This will help you come up with words to the song, but it might also influence the music you are composing. Start vague at first, then get more specific. Or pick a specific object like a car, and make a list of word associations. A good brainstorm can sometimes lead to a full set of lyrics quicker than you think.
Once you start to come up with the bits and pieces of the song, the next step is finding out how they fit together. Experiment with melodies that are in unison with the accompaniment and then in harmony. Think of rhythmic variations. If you get stuck, try playing the same chords or singing the same melody in a completely different rhythm, as that can often help.