If you hear the gas valve open, it should have DSI, direct spark ignition, You should heat the ignitor sparking. If the voltage is low, you may not be getting the sail switch to clear signaling that the air is flowing.
If it is an older furnace, it may have a standing pilot which you have to manually light. But you should not be hearing the gas valve open if the pilot is not lite.
On the heater cover, there should be some instructions.
Let us know the brand and model.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot