Instructions
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.