start up disc full

How do I clear some space up?

Empty the Main Trash.
In iPhoto, Empty Trash.
In Aperture, Empty Trash.
Check that Mail is Emptying its Trash.

Check the number of Movies you have in iTunes, copy the ones no longer watched to DVD.

so on and so forth...

Thanks Kaveman!
what is Aperture? I don't think I have it on this iMac..
I wanted to know if I can use my external HD (with 2TB) to keep my iTunes music library and my iPhotos..?

Sure you can.

Move the iTunes Folder from your Music Folder to the external drive.
Launch iTunes with the Option Key held down, choose the folder on the external drive.

Move the iPhoto Library from the Pictures Folder to the external drive.
Launch iPhoto with the Option Key held down, choose the Library on the external drive.

Once you are sure the copy is ok, you can delete the originals.