Validate

Dear Sir/Madam. I am new to web page design and am currently learning your Westciv coarse I bought. I do not understand the message (below)I get when I try to validate. Could you please help me. 
Regards Don.

 Sorry, I am unable to validate this document because its content type is text/plain, which is not currently supported by this service. The Content-Type field is sent by your web server (or web browser if you use the file upload interface) and depends on its configuration. Commonly, web servers will have a mapping of filename extensions (such as ".html") to MIME Content-Type values (such as text/html). That you recieved this message can mean that your server is not configured correctly, that your file does not have the correct filename extension, or that you are attempting to validate a file type that we do not support yet. In the latter case you should let us know that you need us to support that content type (please include all relevant details, including the URL to the standards document defining the content type) using the instructions on the Feedback Page.