Start With the HVAC Problem You Have
Describe what the system is doing: no cooling, no heat, weak airflow, unusual noise, water, thermostat trouble or another heating and cooling issue.
What to Have Ready
If possible, know whether the issue affects heating or cooling, what the thermostat is set to, when the symptom started and whether the indoor blower or outdoor unit is running. None of that is required before calling, but it can help the first conversation.
Urgent Safety Conditions
Fire, smoke, a suspected gas leak, a carbon-monoxide alarm or another immediate life-safety hazard requires the appropriate emergency service or utility rather than ordinary HVAC troubleshooting.