This might be helpful:
http://bobsguides.com/porting-your-site-to-modx.html
If you use Dreamweaver templates, you can just copy them directly to a new MODx template, move the CSS files, and put the correct path to them into the template. You should be able to preview the site at this point, although your menu may not work and any PHP code on the site will be ignored.
Then you can move the "editable" parts out of the template one at a time, previewint the site after each action.
Content that appears on every page stays in the template.
Replace the main content section with [*content*] and put the content of each page in the "content" field of a new MODx document.
Replace the menu with a call to the Wayfinder snippet.
Replace any content that appears on more than one page (but not all pages) with {{chunkname}} and put the content in the named chunk.
Move any PHP code on the pages into snippets you create and put a call to the snippet [[snippetname]] where the code was.
Be sure to ask questions here if you run into any trouble or confusion.