With this plugin you can handle sidebar contents like posts and assign them from a dropdown menu.
There are several ways to install a plugin in WordPress. For newbies this way is highly recommended:
wordpress-custom-sidebar.zipfrom 'Install Plugins'
Make sure that your theme is supporting widgets. Then add the 'Custom Sidebar' widget in any registered sidebar of your theme. This options are available under 'Widgets' in 'Appearance' menu. For more information look at Widgetizing Themes - WordPress Codex
For more information please follow WordPress Codex - Installing Plugins.