There are two main ways to make money from mobile app.
One: you create an app and charge people for using it. You decide they should pay you through app purchase or in-app purchase. People usually don't buy an app unless it's very popular. So I suggest you offer in-app purchase. You can let them use one part of the app free (to make them like it) and pay money to use the app completely.
Two: you create a free app and allow advertisers to put their ad in your app. You and advertisers will work through a mediate platform. I have used 3 mobile ad platform: Admob, StartApp, Adsota. And I recommend Adsota & Admob. StartApp is ok but the 2 other two just work better for me,