When you first set up your iPhone, you are prompted to log in with an Apple ID to proceed ahead. During that process, you can add only one account to your device whose Contacts, Calendar, Notes, Reminders, Keychain, etc. will be synchronized in the device.
Follow the steps below if you want to add another account, be it Google, Yahoo, or any Email Service, so that you can sync their details to your device.
Once you have logged in with your Email ID and Password, you will be able to choose which services you want to sync. You can also turn on/off the services later by going to Settings>Email>Accounts>”Your Account” and turning on/off the Apps according to your need.