You need to source a theme from WordPress.org approved themes, available from http://wordpress.org/extend/themes/. This is to make sure the theme you use is secure and robust, making it reliable. There are thousands to choose from, so don’t worry.
Before you choose a theme, ask yourself a few questions:
Do I need an image gallery?
Do I need a media player for audio or video?
Do I want to use Slideshare or Scribd?
Do I want to have different types of forms?
Do I want to run a blog, or just have a few static pages?
Do I want menus at the top, or down the left or right side, or no menu at all?
Do I want an ‘image driven’ layout?
Make a few notes and jot down ideas before you start searching, then get ready to bookmark a few choices, to narrow it down. The process usually takes longer than you think, but it’s fun, so enjoy it!