Forum Moderators: open
A DTD is a Document Type Declaration - in short, a way of stating what version of HTML is written on the page. When the validator checks through your document, it must use the standards for whatever version of HTML the document is supposed to be -- and in your case the DTD at the top of your page says your document is intended to be HTML 4.01 transitional.
How to fix this? Look at the error nessages and warnings you receive and address them one by one. Just fix the top few and then try to validate again. If any of them don't make sense to you, try some searches on Google to get an explanation. If you still have trouble, we're here to help each other.