iTunes won't back up iPhone....

There may be a problem with the existing backup. You can delete an existing backup by going to iTunes > Preferences under the Devices tab.

To create a new backup, on a Mac it is done by control-click on the iPhone in the iTunes source list and at the menu window that appears, select Back Up.

I'm not sure about the backup, but we can get you past it to let your iPhone sync. While it says making backup at the top of the iTunes window, there should be a small "x" to the right. If you click that, it'll stop the backup and it will proceed to syncing.

Have you tried uninstalling and reinstalling iTunes yet?

Do you have a large number of photos available in your iPhone's Camera Roll that have not been imported by your computer or if all photos have been imported they have not been deleted from the Camera Roll after the import process is complete? I ask because Camera Roll photos are included with your iPhone's backup. If there is a problem there, import all photos followed by deleting the photos from the Camera Roll after the import process is complete. This will eliminate these photos from the backup and it may make a difference.