I turned on. Or tried to turn on my Tempstar furnace Friday night and nothing happened. So tried the air conditioner and it worked. I don't know where to start. PLEASE HELP!!

Standing pilot or spark ignition, or hot surface ignition.

To be of some assistance we're going to need more information, such as Model and efficiency (80 or 90+), what are you hearing and seeing on the tstat and on the furnace when you have the tstat call for heat. Also what is your knowledge and skill level in diagnosing this problem? Are you just trying to pare down the possibilities for a tech to come in, or do you want to fix it yourself?