if you're talikng about copying selected text anywhere, the easy solution is to use an automator service.
- launch automator
- choose service
- choose 'receive selected text in any application' at the top - should be the default
- use the 'New TextEdit Document' action or the 'Set Contents of TextEdit Document' action, depending on what you need
- save, and run from the services menu
if you want to select text from a specific application then you can use automator or get more fine-grained control by using Applescript, but appllescript doesn't have an easy way to get generic selected text.