One possibility is somebody has determined your Apple ID password and has logged in and set up Family Sharing on your account so they can download anything you purchase. You need to:
Change your Apple ID password - https://appleid.apple.com/
Leave Family Sharing - http://support.apple.com/HT20108
Set up two factor authentication:
Two-factor authentication for Apple ID - https://support.apple.com/HT204915 - "Two-factor authentication is an extra layer of security for your Apple ID designed to ensure that you're the only person who can access your account, even if someone knows your password." Uses a "trusted" device to receive a passcode to verify adding a device.
In general:
If you think your Apple ID has been compromised - https://support.apple.com/HT204145
If you have problems:
Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.