You can sell them to companies, place them on major app stores and sell ads, digital products or sell them directly to clients.
You need to know how to program in any of major programming languages – java, C#, C++, C, web development, among other languages.
How do you make money?
Develop for companies. To make money, you can sell it to them at a price that covers your cost plus profit. In addition, you can charge annual or monthly maintenance fee. Also you can sell digital products or in-app products where you lock some functionality and allow them buy either from app store or from your website. You can integrate payment APIs for various payment channels for example online payments like visa, PayPal.
Develop for app store like windows where users can buy or download free with ads. You can also allow them to disabled ads through in-app purchase.