Measuring the amount of oil a car has in its tank is done by using the dipstick, which is always located somewhere under the vehicle's hood. The vehicle's Owner's Manual should contain a diagram of everything under the hood to help locate the dipstick's exact position.
After opening the hood and locating the dipstick, pull the dipstick out, and wipe it off with a rag. Next, insert the entire dipstick back into it's hole, and then pull the dipstick back out. Now look closely at the end of the dipstick to see what level the oil is at. The dipstick should show clear markings that indicate if there is enough oil or if more needs to be added.