Early on, motorists would buy gasoline at regular retail store, like a general store or hardware store; it was kept on hand for stoves and farm equipment. Some fuel would be measured out into a pail or jug, then poured into the vehicle. Dr. Horatio Jackson and Sewall Crocker made the first cross-country trip across the U.S. in 1903. Between their car's tank and on-board gas cans, they could carry about 24 gallons of fuel; despite numerous breakdowns, a lack of fuel was never really a problem.

Gas stations started popping up around 1905 or so, with several making the claim as being the first. These were little more than sheds that would dole out fuel by the bucket, with little difference from buying it at a general store. It wasn't until the early 1910s that pumps and gravity feed tanks with built-in measuring devices were used at these stations.

If you want to know more about early motoring, I'd highly recommend Ken Burns' documentary "Horatio's Drive." It covers that first cross-country trip and gets into a lot of the difficulties of operating the first cars.