People have known about Mars since ancient times. Indeed, its name comes to us from the ancient Romans who associated it with their god of war (Mars) because of the planet's red color.
Nasa probably found out about mars from going to school. Most of the people who were hired at NASA back in its early years had to be pretty smart, so most of them probably came from good schools. On the other hand, thanks in part to Percival Lowell, the idea that some form of civilization existed on Mars was wide spread during the early 20th century, and it fueled the literary Genera known as "Science Fiction." It is possible that many NASA scientists knew about Mars even before they went to school by reading about it in Science fiction and Fantasy magazines.