About 90% of apps in the App Store are free nowadays, leaving developers to rely on in-app purchases to make money from their apps. To help you add content for sale within an iOS app, Apple requires a series of steps including filling out a contract and sharing your tax and banking information. Once you get the paperwork out of the way, you must configure your Apple ID for in-app purchasing and install a new development provisioning profile.
Once that is done, you can go into the “Manage In-App Purchases” feature in iTunes Connect to creating in-App purchase products. Here is more from Apple’s Developer blog:
Your Admin or Technical users should navigate to the Manage Your Applications in iTunes Connect, select the application for which they want to create In-App Purchase products, then click on the Manage In-App Purchases button in the ensuing page for this application. See the Registering an In-App Purchase section of the iTunes Connect Developer Guide for more information about creating In-App Purchase products.
Once you’ve made the content available for sale, you should test it to make sure that the in-app purchases are working. iTunes Connect has a feature that lets you buy these features with test user accounts. Once you test it, you must follow a series of steps to implement it including signing the application and writing code. Then you should test the app again once it is live. For more details on how to app in-app purchase elements to your app, follow this link.