Getting a WordPress.com API key is helpful for two reasons:

1. It allows you to utilize plugins like Akismet (spam comment filtering) and WordPress.com Stats
2. It allows you to comment on blogs hosted by wordpress.com without having to fill in all of the required fields

To obtain your own WordPress.com API key, sign up here.

Then, since you know how to install plugins, install WP-Stats and Akismet (Akismet may already be installed with your WordPress package, so simply activate) and use your fun new API key to block spam and see your page views!