It is really simple. You need to do a job that not many people can do. An example of job that does not make you rich is a cashier. Cashiers are poor because there are too many people who can become cashiers. Cashiers are too easily replaceable.
An example of a job that makes people very rich is a professional basketball player in the NBA. The reason basketball players in the NBA are super rich is because there are very few professional NBA basketball players and they all generate a huge amount of revenue/profit. The huge amount of revenue/profit that they generate is divided amongst a small number of basketball players. Hence each basketball player can make a huge income and therefore be very rich.
Another way you can become rich is by becoming a CEO or the owner of a very successful business. The reason the owner of a very successful business is very rich is because he has a bunch of wage slaves work for him. These wage slaves generate a huge amount of profit/revenue, and this immense amount of profit/revenue makes the owner of the business very rich.
Of course, it is unlikely you will ever become a basketball player in the NBA or the owner of a very successful business, so let's be realistic. How do you make a decent income but not necessarily become rich? Most people accomplish this by going to university to learn a marketable skill. Then they get hired by someone else who pays them a decent income.
But you can't become rich by going to university. In order to become rich you need to either a) become a sports superstar (like Michael Jordan), become a pop star (like Lady Gaga) or become a famous actor/actress (like Brad Pitt). Or b) you become the owner of a highly successful business like the owner of Walmart. There is really no other way you can become rich, I'm sorry to tell. Anyways, that's all I got to say for today. Peace out and have a nice day.