There are quite a few ways we can monetize an app
The best choice will be heavily dependent on what your app does and what audience it targets. Here are a few options:
Display ads
Premium (pay up-front)
Freemium (pay for extra features)
Subscription (pay periodically for continued use)
Referrals (like with Amazon's Affiliate Program)
These are just some of the most popular monetization methods, all of which are possible on an app. To help you decide which to use just try and figure out what your users are most likely to buy or be interested in.