Composing a song on a classical guitar really isn't any different from writing one for a steel string guitar. I think that the classical guitar and the voice make such a beautiful combination because each has it's own warmth.
As Anthony wrote, finding the melody is the most difficult part.
Once you have the melody, write down the notes in standard notation, tabs, or just letters. Sing your melody and accompany yourself. Write down the chords that you used above the melody, then work out how you want the chords to be played, as chords, to break into arpeggios or in any number of ways.
There are so many different ways to compose songs for a classical guitar.
The most important part for me is to write down the melody. If I don't write it down, I'll forget it by the next time I come back to work on it later. I've lost a lot of wonderful melodies because I didn't write them down at the time.
Music touches the heart, but playing classical guitar can lift your heart and enrich your life.