Begin by picking a topic in which you have interest. knowledge and passion. It can be related to this class, your hobby, your family -- anthing.
Next find blogs that deal with that topic. Search engines can be of use in finding a blog of interest. There are a number of search engines that index blog content. Technorati seems to be best known. Of course, Google has one, and you can do a Google search for others. Global Voices is an interesting aggregator of blogs. You will also find that many blogs have links to other blogs.
Pick a blog that covers your topic well. Read some of the current posts until you find one on which you have an opinion. Either add a comment to that post or send an email to the blogger telling him or her what you think. Better yet, do both.
When you finish, print out a copy of the blog with your comment on it or your email exchange with the blogger. Also state the topic of the blog, the bloggers name and the URL of the blog.