ummm, the "delete" button?
Best thing, is to decide if its worth keeping or not. If not, delete it. If it is, but isn't use much, back it up on a disk, iDisk, CD, ect and then delete the item from the internal hard drive. Store movies/TV shows you don't watch much, on an external drive to free lots of space. You simply have to plug the drive in, click on the file, and its re set back in iTunes.