Setting up a mailing list is very easy using the cPanel interface. The first thing you need to do is log into your client area. Once you have logged in you will be able to then log into your cPanel account by clicking on the cPanel button next to the domain you wish to make the list for.
If you know your cPanel username and password (they would have been sent to you in your welcome email from Colosseum) you can also access your cPanel via www.yourdomain.com:2082.
Once in your cPanel click on the icon labeled "Mailing Lists" from here you can create a new mailing list or delete/change the password/log in to an existing list.
To create a new mailing list simply put in the mailing lists name (this will be the address people will recieve mail from, for example mailing-list@domain.com), and then put in the password you will use to administer the list twice, alternatively you can also use the password generator to randomly create a password for you. Once you completed all of the fields click the Create List button.