Firs you should visit the Content Management area and start setting up your own menu items like About, News, Contact and so on. You can add as many posts (content) as you wish under a menu item. To read more about the content management feature please see Features -> Content Management
The second thing you should do when menus are set up is to look in the Site Layout & Functionality and start setting up your own Blocks (Layout Blocks). Layout blocks are the div tags that you will be able to customize from css and js. Each block can have unlimited subblocks and you can even limit them to show it only for a defined menu and/or language. To lear more about blocks please see Features -> Dynamic Web 2.0 Layout
Once all your blocks are set up and you have the site's design done from css/js you can start adding custom modules/content to your site, if you have any questions please see the Features section.