Building a Magento site can be a long and complex process, involving multiple designers, developers, huge product catalogues, and build times of 12 months or more. For leaner startups setting up a store can be a simple matter of buying and installing a theme, creating a handful of products to test market response, and sending some traffic the site's way using Google Adwords. Each is suited to a different type and stage of online business.
Magento is notoriously badly documented code-wise, which is probably at least partially intentional, since while Magento is open source, it's not altogether community driven.
A logical place to start is the latest version of Magento to play with and the user guide: http://www.mdg-it.com.au/manage/dl.php?type=d&id=16
While the themes on commercial sites are of varying quality, there are definitely some good (and fast) themes around. http://www.templatemonster.com/magento-themes.php is one of the bigger Magento theme marketplaces. http://www.magentocommerce.com/magento-connect/themes.html also has a large collection, including free themes.
If you are a developer or just building a theme yourself, the design guide, while not a complete resource, gives a good grounding in basic Magento principles and the basics of theme design: http://www.mdg-it.com.au/manage/dl.php?type=d&id=17
Some other links, in no particular order:
Official Magento Forums: http://www.magentocommerce.com/boards
stackoverflow Magento tagged questions: http://stackoverflow.com/questions/tagged/magento
LinkedIn Magento Australia group: http://www.linkedin.com/groups/Magento-Australia-4654604
Stack Exchange Magento: http://magento.stackexchange.com/