Welcome to Apple Support Communities!
You can find some helpful information in this article:
Here's how to sign in with an Apple ID on your iPhone:
When you first set up your device running iOS 5 or later, enter the Apple ID you use for iTunes in the setup screens. It will be used for Store purchases and iCloud.
If you skipped the setup process, enter your Apple ID in Settings > iCloud and Settings > iTunes and App Stores.
Have a great day,