For iTunes & iPhoto it's easy. In your Home folder, there is the Pictures & Music folders. iPhoto Library is located in the Pictures folder, move this to where ever you want it. Then start up iPhoto while holding down the Option key. iPhoto will ask where you're library is and there will be a button (amongst others) that says choose or locate (I forget which but you get the idea). From there navigate to where you put the iPhoto Library and select it.
iTunes is the same, inside the music folder will be the iTunes folder, move that to where you'd like it. Open iTunes while holding down the Option key again and do the same.
Make sure you have the external disk mounted before opening these apps.