First of all, here’s what the HTML code for a link might look like:

<a href="http://wordpress.jdwebdev.com">WordPress Expert</a>

The link, when rendered, would look like this:

WordPress Expert

To add nofollow, you would change it to this:

<a href="http://wordpress.jdwebdev.com" rel="nofollow">WordPress Expert</a>

You may be able to find links to manually nofollow by going to your Theme Editor (under Design) and scanning the sidebar and footer files. You should nofollow links to any page for which you don’t care about the ranking. For example, you may want to nofollow links to your RSS feed, since there isn’t much point in that appearing in a standard web search.