How did astronauts survive in their first launch to the space

im wondering if how did they successfully launch their first space travel to the outside of the earth without any idea that what would happen to them and what will they do (since it is their first time to travel outside the earth)

They did have an idea of what it would be like. Lots of experimental craft (some even carrying small test animals) were sent up to collect data.

They made suits to be pressurized and provide oxygen long enough to survive the trip. Pretty straightforward, yet, technically challenging several decades ago.

By drinking Tang, an orange citrus-flavored drink.

JFK decided to have the mission designed by the clever people.