A/C Heater has gone out on my truck i need help to figure out the problem!?

my a/c heater on my 2004 gmc sierra quit working a while back but it came back on and ran for a while and its out again! i know a little bit abt mechanicing but not a lot abt a/c heaters so im doin it myself so i can save money and learn a little abt it! Heres the details all my levels on my main dash controls are off by 1 for instance 0 is 1, 1 is 2, 2 is 3, 3 is 4, 4 is 5, and 5 is 0! you can hear the fan kick on when you play with the dial but no air blows, when im driving over 30 mph it will blow hot when on heat and cold when on ac but very little and it blows harder the faster you drive but when its just idling it does nothing! i think it might be my blower motor or my resistor if this sound like something you have dealt with or you know what it might be any advice is GREATLY appreciated i love learning new stuff so teach me! thank you