just where and what ones do I need?
You'll need whatever documents you use. Only you know whether you can turf last year's tax return, or wipe out your phone directory. If you're not sure on something, back it up before removing it, just in case.
if I place files into document on the desktop the system makes copies of these, why?
Generally, if you just drag a file onto the Desktop, or anywhere for that matter, it will be moved. It's only when you're dragging things from one drive or volume to another that it defaults to copy.
So, you're probably grabbing the file from another drive, or something.
If I trash these the copy does not work ??
Perhaps you are making an alias of the file, which is basically a pointer to the original file. I can't imagine why or how you'd be making aliases, though.
Can I change the icon of Favourites ???
Yep.
First, remove the favourites icon from the finder toolbar.
Then, Show Info on the favourites folder.
You can then paste the new icon in using cut and paste in the Info box.
Once you're happy with the icon on the Favourites folder, just drag the folder onto the toolbar to put it back.