Right-click on the "Outlook Today" icon.
Click on the "Advanced" tab
You will see a path name for where your data is located (a .pst file)
Go to that folder and move the .pst file to a folder in your My Douments folder.
When you next start Outlook it won't be able to find the original .pst file and will ask you for the new location. Navigate to this and select the .pst file that you moved. You may get a warning about should Outlook use the default settings, just answer no and close Outlook.
The next time you open Outlook it will display the .pst file in you My Documents folder. You can confirm this by using the procedure above.
I try to keep all my important data in My Documents folder then all I have to do is Back up the one folder.