Ive done what digimods have said:http://www.digimods.co.uk/tutorials/tutorials.htm

I've lasooed the body of the car, it then says to copy and paste it , every time i press Ctrl+C it says 'Could not complete the copy as the selected area is empty.
Ive tried to click on that layer but as soon as i do that the lasso dissapeares.

 

after you've selected the area you want to copy make sure your on the layer you want to copy then hit ctrl C

when selecting what layers you want to use make sure you do it on the layers window, if you just click on the picture you will loose what you've seletced especially if you have the lasso tool selected.

if you want to get what you've selected back then hit ctrl alt Z to step back (this will only work during a session you cannot step back when you've just opened a recenlty saved document)