90 Heating Problem

'90 88. For a month, it work off and on. Now it only blows cold. Haven't tryed anything yet, not sure what to try. Couple things that come to mind is thermistat, blend door, and clogged heater, but I'm only 19 and I don't really know all the symtoms of everything. I'm leaning towards the blend door, but I just don't know. Thanks for all the help.

Hoses are cold, so I'm thinking Thermistat now. Please tell me if you think I'm right or wrong because on a 3.8, I think its going to be over hundred, more then price will to through away for a high schooler.

Thermostats are cheap. 5-10 bucks.