You'll need to login to the web interface of your router to change the settings. Usually this requires opening a webbrowser and going to an address such as
http://192.168.0.1 but it can vary, look in your manual.
BT Yahoo broadband doesn't technically use a username and password to authenticate. To login, you should just be able to leave the user/pass blank, but if your router complains then use:
user:
firstname.lastname@btbroadband.com
pass: adsl
Make sure your encapsulation is set to PPPoA (VC) and the MTU should be set to 1458
HTH