When I turn on the heater only cold air comes out

SOURCE: Car Heater blowing out cold air regardless of temp setting

you could just be low onwater

SOURCE: Heater blowing cold air.

replace thermostat its clugged,and only allows very little water to go through

SOURCE: 04 gmc yukon heater air cold passanger side.

If this happens when you are idle, and then you get hot air as you drive, most likely you have low coolant and it only cycles as you drive. Get approved dextrom gmc coolat and cap off your coolant it is most likely low.

SOURCE: My heater air cold when turn my heater

have your heater core checked out