Once you plug in the router and connect to it via WiFi you should get a popup that asks for the username and password.
If this happens then simply put in the username and password that has been emailed to you and everything else will be configured automatically.
If you connect via ethernet, or the popup box does not appear, then you will need to configure the connection via the web interface. To do this enter the router IP, 192.168.1.1 by default, into the address bar of your browser.
You will then see the web interface. Click on Sign In at the top right.
You will then be asked to sign in, the default credentials are admin as both the username and password. If the router has been preconfigured the password may be the serial number of the device. You may then be asked to change the admin password, you may wish to do this as it is advisable, however if you do not then click skip.
You should then be back on the main screen of the web interface, logged in as admin. Click the Setup Your Gateway option that will have replaced a picture of the router under the Gateway heading.
You will then see the Connection Configuration page. Check the radio boxes at the top and confirm that ADSL is selected. If not then select ADSL and then click Switch Connection Type.
Once you have confirmed ADSL is selected, leave all the settings as they are and just fill in the Username and Password boxed with the information emailed to you.
Once you have done this click Save and then your connection will have been setup.