Some things are critical for a pilot to know. How much fuel you have for one thing and how close you are to the ground. A fuel gauge is pretty strait forward to read and interpret, much the same way as one found in a car. If the needle is to the right, you have plenty of gas, on the left and it's time to do something about it. An altimeter works pretty much the same way.
Sit in the pilot's seat.
Locate the altimeter. It looks like a round clock face with three needles and a small window where the current barometric pressure is dialed in. It is an instrument in the cockpit that displays your pressure altitude above sea level.
Turn your radio on and obtain the current barometric pressure from air traffic control or ATIS.
Dial that in on the altimeter window. Standard pressure is 29.92, but it could be higher or lower than that, depending on the field elevation and current weather conditions. This will calibrate the altimeter.
Look at the altimeter. Your current MSL elevation will be displayed on the altimeter. The largest hand points to the hundreds of feet, the next smaller hand to the thousands and the long thin pointer to the 10 thousands. MSL is mean seal level and is the standard used by all pilots.