Identifying your blog?s purpose will assist in finding the appropriate wordpress themes in the long run. If you will setup a blog for releasing your views and emotions you should not center much on other aspects like adsense themes or widget themes.
However, if your blog is intended to assist other people you need to consider if a particular theme supports advertisements with no need to do much about the code. You will have to learn if the theme is associated with SEO, Spam protection, RSS and others. There are default themes given on WP themes, however you may also pick your own.