While you can charge for premium games and make money, there are ways to make money through free-to-play games too. One of these ways is by introducing in-app purchases. These can include items that the players can buy for their avatar or amount that they may have to pay in order to upgrade themselves to another level. You can keep introducing new items and levels and open new avenues to make money through in-app purchases. Another method of making money through free-to-play games is to use ads.
There are many ad networks that will be ready to include their ads in your games. You will get paid whenever a player sees the ad, clicks on the ad or installs the app pertaining to the ad, as the case may be. Nevertheless, it is important to decide how and where you want to integrate the ad in your game in the initial development stage itself.