Akismet is a particularly efficient spam filter service that requires an API key to work.
To get the API key, go to the Akismet API key signup page and choose the plan that suits you the most. You'll have to fill a form. Don't forget to write down your email addres in order to receive the API key. If you have a Wordpress account, you can also use your identifiers to get the Akismet API key. If you choose the "Personal" plan, the key will be free and you'll be able to use it on a multiblog.
Enter the API key and save. If the key is recognised, you should get the following message "API key checked". Go back to the filter list to activate Akismet if it's not done yet.