its not too bad. if you get a copy of your itunes library on the external, you can gran the itunes music folder and just drag it into "new itunes" (itunes running on the new computer) and it'll import everything and update permissions, etc. This won't preserve playlists or play counts, however. If you want more than that, you'll have to get familiar with moving the folder to a new user account and modifying the permissions. If you don't know what I'm talking about (permissions) then I would definitely not pursue this.
Your first step for a lot of this should be apple's kbase (apple.com/support) as apple puts a lot of resources into their support structure. Here's a link to get you started http://support.apple.com/kb/HT4527