The WordPress Theme you choose will be the design and functionality for your website or blog. What I mean by this is, the design (or overall look) of this article and blog and website has been determined by the "theme" I chose for this website.
There are tons of themes available on WordPress for you to choose from. You can either use a free theme or purchase a theme to use. I personally use free themes right now, but if I find a theme I would like to use in the future I would definitely buy one. Having said that I would start with a free theme until you are comfortable working with WordPress.
What WordPress Theme Should I Choose?
That answer depends on you and your goals for your website or blog. When you search for themes it allows you to preview it. I recommend doing this, this way you will have idea what your website or blog would look like. Also read the details of each particular theme you are interested in. Make sure it has the features you want to be able to use now and in the future.
For even more great information regarding anything internet marketing related, check out my Blog.