How do I turn this to a touched function?

I essentially answered this in your last post

Part.Touched:connect(function(touchedPart)
    print(touchedPart.Name) -- This is what I wrote before.
    print("Lighting on fire!") -- This is what you wanted... Not very hard to change.
end)

Alternatively, you can write this:

local function lightOnFire(touchedPart)
    -- Do your thing.
end

Part.Touched:connect(lightOnFire)