A template is what controls the look and feel of your web site. The graphical design of the site and the content are separated by the use of a template.
Here are your options for templates:
Use the default templates installed with Joomla or Wordpress
Your site will look like an out of the box site - Not very unique
Choose a free template from one of the many websites offering free templates.
For the price, some of these perform very well and look great
Choose a commercial template.
By commercial we mean you pay for it - you get the advantage of something well written, supported, updated and often with slick results. A lot of them offer many parameters and are very code heavy.
Convert a css template into a template - some programming knowledge required!
Make your own template - some programming knowledge required!