It's a lot easier if the purchased data is in your app and can be unlocked. Then you don't need a server.
However, if you do need a server, at the minimum you should set up a VPS hosted solution, for example over at Linode. This gives you a bare-bones Linux installation. You will have to install all the other software yourself, but Linode has good tutorials.
This is how it works in a nutshell:
1) Your app asks for a list of available In-App Purchase products from the server. So your server needs an API that sends the list of these products back to the app. Then the app sends the product ID's from the list to the App Store to get the store items.
2) When the user purchases an item, the App Store sends your app a receipt. Your app must send the receipt to the server and in return the server will send the downloadable data back to the app. The server should store the receipt in a database table.
There is a little more work if your IAP products are subscriptions or consumable content.
If you have no idea how to do what I just said, then I suggest you hire someone to write this stuff for you. I'm available for contract development and I've written stuff like this before, both for the app side and the server side.
Alternatively, you can use a third-party service to handle all this, such as
Urban Airship - Powering Push Notifications, In-App Purchase, and Subscriptions for mobile applications across major mobile platforms.