where should I start?...no heat!

My 1972 Dart is pretty new to me. One of its issues, is the heat does not work. The blower works and will blow air out of the vents however it is not hot.

Where should I start as far as getting this thing to start blowing some heat? Im in Wisconsin and fall is coming. Ill put it away in late October, but it can get pretty cold at night here soon. Thanks.