Website content theft is a common problem. Quality content can take a lot of time and effort (or money) to create – and it can be stolen in seconds by a child with a computer and a internet connection.
Legal proceedings are the ultimate weapon, the nuclear option, in your armoury against website content theft. But before issuing proceedings, you should consider in detail the other options open to you.
I should say that the title to this post is misleading (at least to an English lawyer). Theft is a criminal offence. Whilst the unauthorised copying of website content will usually constitute a civil infringement of copyright, it will not usually constitute a criminal infringement of copyright. So, it's unlikely that the police or trading standards officers will assist you if someone copies material from your website. There are, however, a number of things you may be able to do on your own, without incurring the expense of a lawyer.
First, you should consider contacting the person responsible for the website asking for the material to be removed. If the website in question does not have a contact email address or HTML form, you can perform a WHOIS search (e.g. at Domain Tools) and write to the email address(es) given.
If that does not work, you should consider writing to the ISP hosting the website. You will usually be able to identify the servers hosting the site (and the relevant ISP) from the WHOIS search.
If the ISP is in the USA, you should make your notice to the ISP compliant with the requirements of the Digital Millenium Copyright Act (see Wikipedia). A list of designated agents for the service of DMCA notices is available on the US Copyright Office website.
If the ISP is in the EU, you should point out in your notice that, as a result of your notice, the ISP will lose any protection under the Ecommerce Directive that it may have had against proceedings relating to the infringing contract.
In any case, your communications with the website owner and ISP should make it clear: (i) what the infringing material is; (ii) where the infringing material can be found; (iii) your full name and contact details; and (iv) details of your rights in respect of the material (e.g. that you the author or an exclusive licensee of the material). The appropriate degrees of politeness, aggression, etc will depend upon the circumstances.
If neither the website owner or the ISP responds – or if neither responds helpfully – that is usually the time to consider legal proceedings.