What is the original source of most of the energy that people use?

The source is stars.

Most readily available energy (e.g. solar, wind, wood, alcohol, fossil fuels) originated in the process of thermonuclear fusion in our sun (which is the closest star).

Geothermal energy comes from radioactive decay heat inside the earth, these radioactive elements were produced in supernova explosions of giant stars more than 6 billion years ago.

Nuclear energy comes from fission of uranium inside nuclear power reactors, the uranium for these reactors was produced in supernova explosions of giant stars more than 6 billion years ago.