Select that which you want to cut (Ctrl-A selects all), Ctrl-X to cut, Ctrl-V to paste. What exactly are you wanting to cut and paste? The entire file? The text of an email?
You can also select part of an email by highlighting with your cursor (or the direction keys and control together). Once you have done this, right click and a menu will come up. Choose "copy."
You can them click "new mail message, and then right click again, this time choosing "paste."