My blog is not hosted by WordPress, instead I installed WordPress using Fantastico in my site’s cPanel. I want to enable Akismet in WP, but I need an API key which means I have to sign up to WordPress? If I do this, will it work with my blog which isn’t a ‘example.wordpress.com’ blog but my own domain?
If this wont work, are there any plugins that will do the job? Every day I’m getting hundreds of spam comments 
Yes you will have to sign up with wordpress.com but when you sign up you can choose the option to only get a username and not a blog.
Once you get signed up, go to your profile and your API Key will be listed.
Another plugin you can use is WP-SpamFree
JVZoo Love
How to get a WP API key
http://en.wordpress.com/api-keys/
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The above link should answer all your questions
References :
Yes you will have to sign up with wordpress.com but when you sign up you can choose the option to only get a username and not a blog.
Once you get signed up, go to your profile and your API Key will be listed.
Another plugin you can use is WP-SpamFree
References :