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As you add more modules, more blocks become available to you. In addition, you can create custom blocks containing your own content.
To create a custom block, you would go into the Blocks admin section. You'll see a drop down menu showing you all of the blocks available on your site for each module. Go down the list and look for "System" and then "Custom Block." Use that option and click on the "Add" button the the right of the menu. The instruction should be pretty clear, but if you have any questions, someone is always here.
To add your own content to pages, you would have to edit the theme.html file in your sites "themes" folder. It's extremely dangerous to mess around with the index.php file, and the index.html files are installed into most of the folders as a security feature.
