The model number ... could it be an NT 24 S maybe? or some other last letter/number. The NT series did not have 3 numbers, only two.
When you take the front cover off do you find a black knob down low, about in the center, which says Pull on it? If so it is a pilot light model and you will need to light the pilot light first. To do that, first be sure the tank gas is on, then check both outside where the gas line comes up and into the furnace, and inside near the furnace for valves which could be there to shut the gas off. The handle of the valve should be parallel to the gas pipe to turn them on.
Then light the stove to be sure that you have gas in the lines at least to the point of the stove.
Then remove the round cap cover you will find on the left side of the combustion chamber, about half way up. With a flashlight, look inside to see where the pilot light assembly is located. Then with a propane gas lighter, bathe that area with flame while pulling out on the black nob mentioned earlier. It might take up to two or three minutes to get gas to the pilot light and have it produce a flame. Once the flame has burned for about 45 seconds, you can release the black knob and see if the flame sustains. If not, pull the knob, and re light holding the knob out longer until the flame sustains.
Only after that is done, can you turn the thermostat up and see if the main burner ignites.
The whole process is a royal PITA and is why new furnaces have electronic ignition and no pilot light.
If you have no center black knob with "pull" on it, you have a newer furnace with the electronic ignition. With those, all you need to do is to have the gas on, and turn the thermostat up and wait and hope that the electronics work, and the furnace will light off.
That should get you started anyway.