Or are you expceting us to write the song for you??
If you have a rhythm already you got a good start...
Next I would go one of two ways:
1. Write out some lyrics... Then sing them over the rythm and adjust the melody and lyrics accordingly.
2. Hum or sing a melody over the rhythm... then take the melody and write lyrics to fit it. Normally this is the one that I would choose since #1 often sounds forced unless the rhythm section is just a very basic chord progression.
