Go to Settings -> Mail, Contacts, Calendars -> Add Account. Select Microsoft Exchange.
When you add a new Exchange account to your iPhone or iPad, include your email address, password, and description. When you select Next, your email configuration will be automatically configured and the information you provided will be checked off. If email configuration is not automatic, enter outlook.office365.com for the Server, leave the Domain blank, enter your email address for the Username and tap Next. Then select the desired services and choose Save.