Get Bob's book.
http://www.amazon.com/MODX-Official-Guide-Bob-Ray/dp/0983619409/ref=pd_sim_sbs_b_1/182-4886514-3947747
Spend as much time as you can just going through these
http://rtfm.modx.com/display/revolution20/Home
http://bobsguides.com/modx.html
You can make a MODx template out of any web page. Choose the page you want to use for the basic look and feel of all pages, and copy the HTML. Replace the content areas with MODx tags. For example, replace the menu with Wayfinder snippet tags, and the main content area with a content tag.
You can have as many different templates as you need, but do try to keep them to a minimum. Perhaps one for the home page, as these are usually fairly complex and unique, then one for normal pages, one for wider pages with charts or forms or galleries. I've gone to great lengths in playing with the CSS to allow the whole site to use just one template, but that's not at all necessary. With multiple templates, it's a good idea to break out the common parts, like the masthead and menu section, into a chunk so that you only have to edit it in one place if necessary.
Gather up all the .css and images used in the template itself (background images and such) and put them in one place, I like to have them in my /assets/ folder in assets/templates/template-name/.
Adjust the paths to the .css and .js files in the template to point to where you put these files. Make sure the css file is pointing to its background images relative to its own location. That's one thing that frequently trips up new MODx developers; everything MODx itself uses needs to have their paths relative to the location of the index.php file, since everything is run from that. The CSS file is requested by the browser separately, though, so any paths it uses need to be relative to itself. Likewise .js files.
As you work with MODx you will see different ways to use different elements to simplify your life. Find out about Template Variables, and use them liberally. The MIGx add-on, which extends Template Variables capabilities, will be a real life-saver.
http://rtfm.modx.com/display/ADDON/MIGX
Did I say to get Bob's book?