By default, your mailbox opens with the Inbox view. All messages in your Inbox are displayed in the Content pane. You can change the initial view to be any of the folders, searches, or tags in the Overview pane. For example, if you would like to see only your unread messages when you open your mailbox, go to Preferences>Mail>Default Mail Search and in the field type the search as is :unread.
Or, to see mail tagged with the "ToDo" tag you created when you log in, enter the search as tag:ToDo. To see mail in a folder other than your Inbox, enter a folder search, for example in:Work.
Use quotes if the folder or tag is more than one word. For example in:"Unread Email."