<p>How do i take a HTML site that currently has no doctype declaration and make it W3C compliant?</p>

First of all: think about
why
you need to make it W3C compliant. If it's just for the little badge, then it's not worth it.
If you do have a good reason (and there are plenty), then this is what I'd do:
Start by choosing a doctype. From the sound of it, HTML 4.0 might be the way to go. Run your page through a validator and then look at the error report it gives you. Go through it one by one, fixing each error. Usually the error report will tell you a good deal of information about how to solve each problem and give you the relevant links to the specification.