The app store is associated with your Apple ID.
This is usually the account you first signed up with when you bought your iPhone, but it's fairly easy to change your email address if it needs updating:
- Log out of all services that require Apple ID
- Select My Apple ID, manage, and the log in to this section
- Hit the "edit" button and update what is marked as your "primary email"
Following these steps should mean you'll have the correct email address from now on!