Money is worth something because at least more than one person agree that it does. Otherwise, if you wanted to use it to buy something, nobody would accept it. That agreement between two people, the one selling and the one buying, is what makes the whole thing to work.
Just like when you play a game, there are rules. Those playing the game accept and play by those rules. If you just decide to change one such rule by yourself, you’re not playing the same game any more.
For example, if you’re playing football, and suddenly decide to pick up the ball with your hands and run to the goal, your playmates won’t probably like it. You all had an agreement to play with your feet when you said “let’s play football”.
Money is also an agreement. We can also call it a protocol. Just like a game, it has rules that everyone in the market agree to follow when they use that money. For example: the name of the money is “dollars”, so if you ask for a price of 5 of them, you say “it costs five dollars” and the other person is going to understand that you’re not talking about 5 goats or something like that. Other rule is using paper rectangles with a special size, ink, and design that will represent amounts of dollars: the bills. Another is that the money can be divided in cents, so you can have prices that are less than 1. You get the idea.
So in order to create money, we need to come up with the best rules we can. Those rules will make or break the game, and make people want to play or not.