Akismet is a comment spam filtering service. Comment moderation is a time consuming task, and Akismet can save you hours. Akismet will catch spam comments before it lands in your moderation queue as pending. This allows you to focus your energy on moderating comments by real users. Even though it's activated on your website, you still need to set up the API for it to work.
1) You can find the Akismet Anti-Spam plugin under Settings. Once there, you can either enter your API if you already have it set up or you can activate it.
You should be fine using the free version.
2) If you already have a WordPress.com account, then you can click on the link I already have a WordPress.com account. If you don’t have a WordPress.com account, then simply fill out the details and create one.
3) Once the authentication with WordPress.com is complete, you will be taken back to the Akismet website. There you will be asked to provide some user details along with the payment information. If you picked the free plan, then simply drag the price amount to $0.
Click the continue button and you’ll be taken to the screen that shows your API key. You will also receive an email from Akismet containing your API Key.
4) Copy your Akismet API key and paste the API key into the Akismet settings. Rest of the options on this screen are optional. Once you click on Update Options button, Akismet will now verify your key and show you a success message.