upon starting my 2001 gmc sonoma, this winter. After a short warmup i turned on the heater and nothing. No air blowing at all hot or cold. what should be the first thing i should check?

Heater blower inop. Check fuses. Then jumper the wires if necessary to run the blower with a 12V power source you provide. Then check power in vehicle harness is provided to the blower. And ground is provided on the other wire. Many vehicles also achieve blower speeds that are less than high or max by using a resistor block that has preset resistors for each speed. the resistors are mounted in the airstream of the heater case to let the passing air inside the case keep them cool. The resistor block may have a thermal limiter. When it breaks from overheat, you must replace the blower and the resistor block after testing and restoring connections and the problem repeats after testing. The switch connection may be checked if no problem found yet, and the function control should also be inspected. If access and testing must vary from this, the service manual must be consulted because many systems are possible and in use.