How to remove default Joomla menu items, menus and banners
To remove a whole menu from your site's frontend, log in to your backend (admin panel) and go to the Module Manager (Extensions>Module Manager). Find the module for the menu you want to remove, mark the checkbox in front of it and click on the Unpublish button in the upper right corner.
For example, if you have installed the sample data and you want to remove the About Joomla! menu, find the module with the same name and unpublish it. If you haven't installed the sample data, only the main menu will be activated, but if you add any new menus you can remove them in the same way.
To remove menu items go to the Menu Manager (Menus>Menu Manager), select the menu from which you want to remove items (e.g. main menu), and in the manager for the particular menu unpublish the items you wish to remove.
To remove the bottom banners go to the Banner Manager (Components>Banners) and unpublish the three default banners there (Shop 1, Shop 2, Support Joomla). This, however, will only remove the images from your site's frontend. The gray area and the titles (Contribute, Books, Shop) will remain. To remove them too go to the Module Manager, find the modules Book store, Shop, Contribute and unpublish them.
The above is valid for those who have installed the sample data. If you haven't installed the sample data, only the gray area and the title Books! will be present on your site's frontend. To remove them go to the Module Manager and unpublish the module Book Store.
For more information and examples read the tutorial on removing default menu items, menus and banners from your Joomla site.