The quickest and easiest solution is to backup your entire drive using Time Machine.
Whatever iTunes Media and iOS backups you have stored locally on your computer will be backed up.
For more information on setting up a Time Machine backup, check out:
Use Time Machine to back up or restore your Mac
However, if you are looking to manually back up the iTunes files, simply drag and drop the iTunes Media folder. From Finder window, select your user account (home folder) - click on the "Music Folder" - then "iTunes" and you will the see "iTunes Media" folder. Drag that folder to the external HD.
For finding iOS backups locally, check this out:
Locate backups of your iPhone, iPad, and iPod touch
And drag that folder to the external HD