If you don't have your sights set on being a commander or a pilot, all you need is a bachelor's degree in a relevant field and three years of professional experience, according to NASA.
Your degree can be in anything from biology, to nursing, to math.
Sounds pretty easy! But then it gets tricky - with qualities that you can't easily fix. NASA has tons of requirements beyond education.
You must have 20/20 vision or at least vision that's correctable with a prescription to 20/20 in both eyes. You have to be somewhere between 58.5 inches and 76 inches tall (between about 4'9" and about 6'3").
You have to pass a NASA-specific physical and psychological evaluation. A more advanced degree is also helpful, and it can take the place of the three years of experience requirement.
And of course you have to survive the training. Lots and lots of training.
It includes more studying, military water survival drills, and rides aboard NASA's famous Vomit Comet - a reduced gravity aircraft.
You need at least a bachelor's degree, but an advanced degree is more desirable. You also need to have logged at least 1,000 pilot hours in a jet aircraft.
And of course you're subject to the same physical and psychological requirements and training.
If you make it through training and become a fully fledged astronaut, it will likely take years before you're selected for spaceflight. Once that happens, you have another couple years of training before you leave the ground.
These are NASA's rules - other space agencies have slightly different requirements and training regiments.
Spaceflight is incredibly risky and expensive, so it's no wonder space agencies have such strict requirements and intense training.