A DuckDuckGo search for “html validator” as well as a Google search will yield many results. Note that you should first choose a version of the HTML or XHTML standards (such as HTML5 or XHTML5) and use their correct Doctype to indicate that you are using them. Different HTML pages (even ones on the same site) may adhere to different versions of the standards, and validate accordingly, as long as they specify the appropriate doctypes.
HTML Tidy - a library and a command-line program for validating and fixing HTML. It is recommended to run it with all warnings displayed.