Well, Roundcube doesn't do email account administration. You'll need to set up a new email account at the actual email server first (either through your web control panel (cPanel, Plesk, etc.)) or manually on your server.
Then you can open roundcube and just log in. Roundcube can automatically add a user to its database if it's set up to do so; otherwise, you'd have to add it to the roundcube database, and then log in.