Answer: Ride height is the space between the base of your tire where it touches the ground and the underside of your vehicle.
Your manufacturer specifications, found in the manual, are the place to begin. Of course, you may be going off spec for a reason--lowering for handling and appearance or raising a truck for offroading--and in that case it becomes even more important to know exactly what you are doing.
Make sure you know where the measurement is supposed to be taken. Check the manufacturer literature that came with your vehicle. You might be measuring from the rocker panel (lowest body panel betwen the wheel wells) to the ground, or the fender lip to the ground, or from the bumper to the ground. Some manufacturers may recommend measuring from the frame rail to the axle or a suspension control arm. Obviously changes in the wheel size and tire size will affect the measurement.
Use a retractable metal tape measure. Have the car passenger load as it usually is (ask a friend), and an average tank of gas, say half a tank. When you get the measurement WRITE IT DOWN, both the measurement and the details about how you took it.
Ground clearance is almost the same thing as ride height, but it is measured simply from the lowest part of your car to the ground. Know the lowest part of your vehicle. The minimum ground clearance might be anywhere, but it is often the exhaust system, or the bumper, or plastic shielding, or the differential housing, or a chassis brace. Once you have confirmed the lowest part of your car, use calipers to get the measurement. WRITE IT DOWN. How to Make Your Car Handle by Fred Puhn is an old but good book with some ideas about measuring and adjusting ride height.