A taxi driver makes money after he successfully delivers the passengers to their destination. The more passengers he sends to their destination, means that he makes more. It is that simple and if you are not able to find lots of passenger then your whole trip is not profitable.
In this game, you would need to do lots of things other than send passengers to their destination. You would need to meet the required task for that level. Your time is limited so make haste and don't waste your time. As you progress in the game, your task will vary and it will be much difficult than its predecessors.