This is a good question. For some, they are born with it and will always rise to the occasion. Others have to work thru some rough patches before they figure it out. But a lot of it has to do with having confidence/developing it. So if you are brought into a tie game with bases loaded and no outs you should be thinking this is my mound, they can't touch me. Not- if one of them gets a hit we lose the game or if I throw a ball here I'm in trouble. Confidence is key.
The only way to prepare is by getting exposed to uncomfortable situations. If you're a younger player, spot play for some older teams when you can. When 2019 was an upcoming freshman, he played 16u. It helped him get used to playing with bigger, faster, better players who wanted nothing more than to crush the little 8th grader thinking he could hang. When he was a freshman his first start was against the #1 team in the state on 20 minutes notice after the starter hurt his elbow warming up. Thrown right into the fire.
Its not just baseball. The top athletes in every sport are mentally tough and when they take the field they genuinely believe they can't be stopped. In order to get the nerves and discomfort out of the way, you have to put yourself in stressful situations and work out of them. Don't worry about the result, focus on how you're going to get this out, what you want out of the AB/inning/game, etc.