How did astronauts know what to wear in space?

I'm curious to know, before they made their first space walk or first landed on the moon, how is it they knew what to wear? How did they know if it would be hot or cold? How did they know about the moons gravitation? How did they know what to make the suits out of? How did they know about the radiation levels? How did they know what type of materials for the rocket would withstand all the intense changes, speeds and everything? I'm very interested to get the answers to these questions as well. I have been reading and have not really come up to anything that would indicate just how they knew this stuff in advance. Perhaps I have overlooked something? Please serious or thoughtful answers only. I appreciate your time into my questions, I am only human.