W3C (World Wide Web Consortium) - Home page for the whole W3C
There are different sections of the W3C website, depending on which technology you are interested in. Down the left of their home page there is a list of links to the different sections.
To start with, you would want to look at the following sections, as they would help you understand basic errors shown in the results when you run your pages through the
W3C Validator:
HTML 4.01 Specification
XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)
If you (or your employer) need to know about accessibility:
Web Accessibility Initiative (WAI)
One of the previous posters mentioned having to edit scripts they want to use that did not pass validation. If you want to add scripts you found somewhere on the net and want you web pages to validate, then you are going to have to learn that particular scripting language too.