A neat built-in plugin that comes with every installation of WordPress is Akismet.
What it does is filter comments that are made to your website as compared to a master system on the akismet servers. This will catch the vast majority of comment form SPAM. You can read more about the Akismet service on their website.
To get it working, go to the Plugins panel and Installed. There at the top of the list is Akismet. Just click Activate.
You’ll be sent an email with the API Key for Akismet, which you enter by clicking Plugins and Akismet Configuration, and pasting the code into the field near the top and clicking Update Options. If all goes well, everything will go green!