A company determines how much it thinks it is worth (i.e. $1 billion dollars) and then splits that into a certain number of stocks. If it wants it’s shares to cost $100 each, then they offer to sell people 10 million shares (10,000,000 shares for $100 each is $1 Billion).
If people think the company really is worth that much, or is worth even more, or can become more valuable in the future, they will buy those shares at $100 in the hopes that they can sell them for more later.
If they don’t think the company is worth that much, they don’t buy it, and wait to see if the price comes down.
If 1 million people buy 10 shares each at $100, they each own 0.0001% of the company and the company gets their $1 Billion dollars to them spend on research/products/capital investment/CEO Bonuses, etc.
If the company then becomes worth $2 Billion dollars, those shareholders now own 0.0001% of $2 Billion instead of $1 Billion.
Or the company could fuck up and go lower in value where the shareholders will lose value.