One of the most common worries that I come across when someone wants to set up a new blog or website is about the theme. People get worried about how can they know exactly what they want their blog to look like before they start putting in the content. The great thing about WordPress themes is that you can change them whenever you like. Each theme (or template as it were) has different layouts and functionality, so if you change your theme you might have to tweak things a little, but on the whole a lot of them can be changed with minimal effort.
If you have gotten as far as thinking about themes – you probably have a rough idea of what you want your site to look like. I would recommend that you browse the themes available for free in WordPress and find one which you think matches what you had in mind. As long as it is vaguely on the right track then it’s a great place to start.
Even if the theme is exactly what you wanted, I highly doubt that in the lifetime of your site, you will never want to change it so pick a free theme and get playing around!