Open the "App Store" app
Click the search tab along the bottom right.
Search for 'Outlook'
Click the top result called "Microsoft Outlook" and will have a blue outlook icon. Click the button labelled "Install" to install the app. If you have downloaded Outlook in the past this icon will appear as a cloud icon with an arrow. Click that instead.
Once the application has downloaded you will be able to find it either on your home screen or by clicking "Open" on the app's page in the app store. You will then be prompted to add your account. Enter your email address and click "Add Account". You will then be asked to enter your password.