Whenever you find that your content has been copied, don't wait to take action but at the same time, don't jump the gun by shooting off a legal notice right away. That would be your last resort and before that, you need to try a few easier methods to sort out the problem.
You could begin by sending a mail-in polite but firm language so that the copier gets the message that he's dealing with someone who knows how things work online. Get a screenshot of the copied content on his webpage and highlight it in red before taking a screenshot of the page and saving it in either PDF or JPG format.
Attach the screenshot in the mail along with another screenshot of your webpage from where the person has copied the content and also, share the link to that particular page from your website.
Wait for about a week and if there's no reply, then your next course of action will be to shoot off a legal letter called C&D (Cease and Desist) notice which is actually the first warning of the impending legal action that you would be initiating. Usually, this notice is good enough for the content thief to fall in line but if for some reason he continues to ignore your warning, you need to turn on the heat a little more.
The name of your next weapon is DMCA (Digital Millennium Copyright Act) which is a harder warning. Both the C&D and DMCA notice has legal implications and they have specific formats to be followed. There are very few content thieves who would have the courage to ignore a DMCA notice but if he persists with his act of ignoring your second warning, the next stage would be a withdrawal of access to the website where your content has been wrongfully and illegally used.
This will be done in accordance with the provisions of the DMCA notice. That will more or less settle the issue in your favor following which, you can create more problems for the content thief.