Even when musicians made their own steel drums from empty containers, they were not simply beating on a found object. The sides of the container were cut off at a particular depth, which depends on the range
of the instrument. Lower-sounding instruments have deeper sides, while higher-sounding instruments have shallower sides.
Then the pan was beaten with a hammer until it was the shape of a shallow bowl. Sections in the bowl shape were marked off by hammered grooves, and then each section beaten (from below) until it was tuned to a specific pitch.
The modern, manufactured instrument is still designed much like the traditional pan. Steel pan drums come in several different sizes, with different numbers of pitches that can be played. The tenor pan (the highest-sounding, also called the ping pong or lead pan), which generally plays the melody, can play a chromatic scale over more than two octaves. The bass pan, on the other hand, has only three or four notes. Other pans, which include the guitar pan and cello pans, are tuned in whole tones or chords. These are often played (by a single player) in sets of two or three: for example, double guitar pans or triple cello pans.
Here is an animation that explains how the notes on a tenor pan are arranged, and then allows you to "play" a steel drum. Audio recordings from a steel drum concert can be found at the University of Illinois School of Music's on-line Media Center.