The Office 365 core email service is Outlook.com, formerly known as Outlook Web App. Sending an email from Outlook.com is very easy. Just click on +new mail, and as you type in the name of the recipient, Outlook will offer you suggestions. Once you have entered the names of your recipients, type out the body of your message and add attachments such as documents or images if you need to. Click on send and your email will be delivered to the recipients.
Take some time to explore Outlook.com and discover other useful email functions.