One type of Joomla!® extension is the template. There are at least two kinds of them, website templates and administrator templates. Here we will focus on website templates. Several templates are included with the Joomla!® installation archives and there are extensive tutorials available in the documentation and from other web-based sources. The included templates are well designed and studying them will provide the new user with valuable insights into template design and customization.
An internet search for "Joomla Site Templates" will return an enormous list of templates available for download, both free and paid. The templates available vary widely in cost and quality. Some clients of ours have invested considerable sums of money in templates, and have turned to us to have them customized. The down side of a purchased template is that you or your prospects stand a pretty good chance of seeing your template duplicated on other websites. An alternative to purchased templates is making your own. You don't have to start from scratch, although you certainly could. There are products out there called "Template Frameworks" that make the process considerably easier.
If you search for "Joomla Template Framework", you'll see a dozen or so of them mentioned, and some are quite good, others not as good.
As of this writing, our favorite is the Gantry Framework. We find it powerful, and easy to customize, and we think you will as well. We start with one of the free Gantry templates, Hydrogen or Helium, and go from there to generate a custom template from your design, or one which our design consultants have done.
You may find it odd that the labor required to generate a custom template from your design actually costs more than a commercial template. Sorry, that's the price one pays to insure that none of your prospects see another website exactly like yours.