You make money once you delivered your client to their destination. If you have more client then you would make more money. That is the whole concept of being a driver, but being a driver is not that easy since you need to have the skills, and the route you are taking in the city, so you need to be familiar with the place. Otherwise, you would just lose a lot. For some, they make money from taking the customer to a lot of place since the meter is still going as you drive your taxi, which means that you could ask for a higher fare from the customer. However, you are an honest guy and you know that you could make more if only you could deliver more customer to their destination as fast as you can.
In this game, you can't make more money if you are too slow since the value of the fare decreases as time passes by. You need to deliver the passengers to their destination as fast as you can, so that you can earn more. The number of passenger that you need to deliver to their destination depends upon the level. You can use the money you earn to upgrade your taxi.