Okay so follow this procedure to move your iPhoto librarie(s) to an external hard drive.
The iPhoto library can be located anywhere, but is usually found in the user's Pictures folder. Drag the folder called iPhoto Library to the external hard drive. This actually makes a copy. Do NOT delete the old one yet.
Launch iPhoto while holding down the option key on your keyboard. It will ask you which iPhoto library you want to use. Point it to the copy that is on the external hard drive. Test the use of this library fully, making sure all your pictures are there and it works as you expect.
Once you are satisfied with the newly moved library, you can move the old library (the one on your internal hard drive in - presumably - your Pictures folder) to the Trash and empty the Trash.
Repeat ths above procedure if you have more than one iPhoto library, moving each to the external hard drive, testing it and then deleting the old library.
A similar procedure can be used to move the iTunes library.
PS You should also consider backing up your pictures to more permanent media such as CDs or DVDs. This provides a backup in case of hard drive failure.