Hey mate that is really easy...
just get your hosting account and log into cpanel. There u will find fantastico and softalucus i guess. Just click on anyone of those. Then on that page, select cms and within that find joomla and install it. Thats it you have ur joomla site. To change the template, first download the zip file of the template. There are number of joomla templates available for free. Just google out free joomla templates. After u have the zip, open joomla backend and then go to install/uninstall in the dropdown menu. There u upload the zip file and then go to the template manager to set it as the default