This indispensable plugin is installed with WordPress by default, but you need an API key to register and use the plugin. You get this from the Akismet webpage.
When you activate the plugin, an alert will be displayed at the top of your WordPress dashboard pages informing you that you need to register it with the key. Click to do so.
When saving your settings, be sure to select the option “disable spam older than a month.” If your blog is popular, you’ll be getting a lot of spam and won’t want to keep it.