This is sort of easy.
Select the file and then Command-C will put the path to the file in the clipboard. If you then paste into the Terminal you will get the entire path, unfortunately most other apps you will just get the filename.
If you want the full path then you can create an Automator Service to do it easily :-
Create a new Automator service and change the 'Service receives Selected' to : Files or folders And the in to the Finder application.
Add a Copy to Clipboard Action and Save the Service.
You can now select a file, right click to get a contextual menu and select your service in the Service submenu.
The path will be in the clipboard ready for you to paste. You can even set a keyboard shortcut for it if you like.
Originally I used a Run Applescript Action to get the Posix path to the file and pass it on to the Copy to Clipboard Action.
But I discovered all you needed is to have the selection passed directly to the Copy to Clipboard Action.
This improved on the Applescript because the Applescript could only deal with one item being selected as written.
But now you can select multiple files and get multiple paths in the clipboard without writing any code.