The word "astronaut" comes from the Greek words meaning "star sailor." Astronauts come from all walks of life but they must all go through extensive training before they are ready to head into space. In order to become an astronaut, you have to have a master's degree in a STEM field, have several years of related work experience, and be physically and mentally strong enough to pass a long-duration flight physical. Astronauts must then train for two years before they are ready to go to space.
NASA's first class of astronauts were selected in 1959, and they are known as the "Mercury 7." They included: Alan Shepard, Gus Grissom, Gordon Cooper, Wally Schirra, Deke Slayton, John Glenn, and Scott Carpenter. So far, NASA has selected 360 people to become astronauts.
Source: NASA