Usually your first step is to contact the offender, either by his contact form or his email. Send him what is known as a 'cease and desist' letter, which tells him that he is in violation of your copyright and if he doesn't take your work down, you will take further action. Don't go into detail. He doesn't need to know what you will and won't do. This is just to give him a chance to comply. Put a time limit. I suggest no more than 72 hours. If he hasn't taken it down by then, contact his web host. There you will probably need to file a DMCA request. There are forms in several places that can help you get started. Additionally, different web hosts treat DMCAs differently so make sure yours complies with the conditions his web host has in their terms and conditions. If you need to contact the web host, at the same time contact all major search engines. Again, you will probably need to file DMCAs with them according to their terms and conditions.
If he has marketed you content to article sites, whether free or paid, you can also contact them and alert them to his plagiarism.
It is a LOT of work to recover stolen content, but I have done it several times for both myself and for clients and I think it is well worth the effort. Good luck.