Install WordPress Themes
When you install WordPress, there is a default theme, i.e. a default design that will appear. Likely, it is the twenty-eleven theme.
If you are not graphically sensitive, this theme may be fine for you. However, you may want to install a WordPress theme that is more aesthetic or one that is more functional. For example, some themes have designs that are optimized for different purposes such as maximizing clicks to Adsense or generating comments from readers or getting readers to subscribe. If you are focused on one of these or other objectives, you'll want to install a WordPress theme best designed for your objective.
Where to get WordPress Themes
Most WordPress themes are free. You can install WordPress themes of many varieties and designs from this collection: Http://wordpress.org/extend/themes.
Another option is to subscribe to a library. These are privately designed themes but you can subscribe to a collection of say 500 for under $50 such as here: http://www.wordpressthemes.com.
Yet another option are themes that are specialized such as one with a lot of custom elements that help double or even triple the clicks you get from Adsense such as the HeatMap theme which I use on this blog (disclosure- I am a Heatmap customer and affiliate and they compensate me for referrals, as they will you).
You can also take any theme and personalize it by changing the header (in your WordPress dashboard), the large graphic which appears at the top. You can find headers by doing a Google image search and setting the size of the graphics you search for to match the header size in your WordPress theme.
Yet other WordPress themes you install will permit personalization by allowing you to add or delete specific elements (e.g. menus, widgets, page types) so that you can utilize the basic elements of the theme you like and then make it your own.