Open Settings on your iPhone
Tap Apps
Tap Calendar
Tap Calendar Accounts
Tap Add Account
Select your calendar service
Sign in to your account
Turn on Calendar sync
Open the Calendar app
Wait for the calendars to update
Check that your events appear in the Calendar app
If needed, go to Settings
Tap Calendar
Tap Accounts
Select the account
Ensure Calendar is enabled
Refresh the Calendar app if events do not appear