Carnegie got his first job at age 13 as a bobbin boy in a cotton mill for $1.20 a week after his family moved to the US. He first made his wealth from managing railroads, but eventually moved into steel. His Carnegie Steel Company transformed steel production in the US, and was the largest of its kind in the world by 1889.
In 1892, the company tried to lower wages at a steel plant, but the employees responded with the Homestead Strike. Carnegie was away at the time, but did not escape criticism.
He was also active in philanthropy, establishing the New York Public Library, Carnegie-Mellon University, and the Carnegie Endowment for International Peace.
Bonus fact: When he was young, Carnegie took advantage of a library for working boys, which had been opened by Colonel James Anderson and was super rare at the time. He was so thankful for the library that he vowed to help others in a similar fashion if he ever made money.
Source: Biography, Carnegie.org, Andrew Carnegie by David Nasaw