The best place to start is in the Tutorials section of the site: http://doc.silverstripe.org/doku.php?id=tutorials
Theming is relitvely easy if you have HTML and CSS knowledge. You will find in your default installation (if you installed using BlackCandy theme) two files called /themes/blackcandy/templates/Page.ss and /themes/blackcandy/templates/Layout/Page.ss. These two files are your basic Views (the 'V' in 'MVC') and hold all the HTML and variables that display on your site.
Before delving into themes, I suggest you go through the tutorials first so you can get a good primer on how Silverstripe works.