How do you become an astronaut?

How do you become an astronaut?
Within the next few decades, humans could be leaving their footprints on the Red Planet! That's the plan, as NASA continues to prepare to expand human exploration in the solar system. But what we want to know is: How do you become an astronaut?
To find out we visited Merritt Moore, quantum physicist from The University of Oxford, to ask: How do you become an astronaut?