Alaclek, this has been addressed sooooo many times... but here’s a link to my tutorial on converting an existing html/css page to a MODx template:
Basically, just make a folder in /assets/templates/newtemplatename to hold your css files and template-specific images (such as background images for the CSS). Then copy/paste the raw HTML of the page to a new template in the Manager, adjust the link to the CSS and any images linked in the template itself. Then replace the content, not the structure, with MODx tags such as [*content*] so the content of each page will be inserted into the desired spot in the structure. Also replace menus with [[Wayfinder? &startId=`0`]] snippet calls to build dynamic menus based on the documents in the specified "startId" document/container.
Other than using MODx tags for dynamic content, a MODx template behaves exactly like any flat HTML document.
TVs are an extension of the basic MODx content tags ([*pagetitle*], [*longtitle*], [*description*], [*content*]). They allow you to make up values you want to attach to documents, such as classification tags, images, price and other details for a "catalog" page, or anything else you might want to have for your pages. They actually go far beyond this simplistic definition, but it’s a starting point