You can create folders in your emails to help your organisation. 

Look on the left-hand column of your emails. 

To create a folder, right-click your email address in that column and select "New Folder". Type immediately to change the name of that folder.

You can also create sub-folders in your folders to further expand your organisation. To do this, right-click the folder in which you want to create your sub-folder and select "New Folder". Now, you can hide this folder by clicking the arrow on the left-hand side of your main folder.

For more information on Outlook Folders, check out this page from Microsoft.