Login to your 123-reg Control Panel and then scroll down to the Web Hosting section and then click on the manage button for the Web Hosting section.
This will show you an overview of all of the packages you have in your account. Suggest the package you wish to use by clicking on the Manage button.
Select the domain name you wish to use from the drop down box provided, or by typing the domain name directly into the search box. Then add the email address you wish to use as the admin email address into the box provided, and click the create button.