In order to setup your MySQL on a shared hoster you have to know your logins to your control pannel. Most of the shared linux hoster use CPanel, plesk, virtuozzo etc or similar softwares to give clients access to their domains. Within your control pannel will be option for databases.

Once you open up the databases you could setup users, databases and user priviliages for individual databases aswell and on as needed basis.

Joomla is an opensource content management software and you can setup the whole scenario through their own provided setup pages. One caution at this point will be the naming convention of the databases and the usernames, as some shared hoster will put you hosting username as a prefix to any of your databases and usernames.

DNS propagation can take anywhere from 24 hours to 72 hours.

hope to have answered your questions.