You can visit the webmail service (Roundcube) for your site by putting "webmail" infront of the main part of your domain name. For example, if you had the domain name example.com, you could visit your webmail service at http://webmail.example.com -- you should not include "www" anywhere in the address.
Your webmail username and password are the same as the ones you would use when setting up an IMAP account in a desktop mail client. If you're not sure what these are, you can find them in the hosting control panel (Virtualmin) under Edit Users.
IMPORTANT: The first time you use webmail you will need to tell it your real name and your email address by going to Settings > Identities. If you don't do this, people won't be able to reply to emails you have sent them using webmail.