The d20M SRD can be used to help figure out what the base torch damage should be:
And then the regular d20 SRD for the fire damage:
The PHB explains that improvised weapons have a 10' range increment, and the threat range for unspecified weapons is 20/x2.
Here's one possible solution using just those written rules:
Strike Blow with Torch, -4 melee or -4 ranged, 1d3 damage (possibly 1d4 depending on your view of normal torch size), 10' range increment, 20/x2 criticals, if struck the opponent rolls DC15 Reflex save or suffers additional 1d6 fire damage plus is now considered "on fire"