Separate the obvious problem items first
Before requesting a quote, note whether the garage contains paint, motor oil, batteries, tires, propane, chemicals or other hazardous or regulated material. Those items may require specialized disposal and may not be accepted with a normal load.
Estimate volume, not perfection
You do not need an exact cubic yard measurement. A few wide angle photos and a rough description, such as quarter garage, one car garage wall, pickup load or half truck, can be enough for a preliminary quote.
Decide what stays
Mark keep/donate/remove areas before the crew arrives. Clear decisions reduce labor time and prevent accidental removal.