You set up default, by typing default in search. Then going to the appropriate setting in default programs. .
But I do not know if you can do this with Yahoo
You could set up the email client. To use Yahoo on the client you may need the pay service.
Yahoo is web mail; however, I have heard that you may be able to use it with Yahoo Messenger.
If you want to set up a client go to Windows Essentials and download Windows Live Mail. http://download.live.com/