First, login to your wordpress administration panel and then click on 'Appearances' and then 'Widgets' from the menu on the left.
If you are using the Gantry Framework then you will see the default settings that are used throughout your website. To have a look at what widgets are used in the sidebar of your website, simply click on the arrow in the dark grey 'Sidebar' box on the right.
If you would like to add a widget to the sidebar you can drag and drop it onto the sidebar. To rearrange the order of the widgets in the side bar you can simply rearrange the order by clicking and dragging the widgets.
Most of the widgets that you add have numerous options that you can set, such as the widget title that appears on the website. To edit these settings click on the grey arrow on the side of the widget, this will show all the options that you can set. From this view you can also delete the widget altogether by clicking the 'Delete' button.
So far we have been looking at the default widgets for your website, but there are some pages that have overrides which show alterntive widgets for specific pages. The frontpage of any RocketTheme is usually a good exmple. To edit these widgets you first need to change the view so that you are looking at the overrides page. Todo this click on the 'Defult Settings' buton at the top of the Widgets page, and then click on 'Frontpage'.
If any of the grey boxes in the right have a green tick in them, then an override is present. Simply change the widgets here as you would in the defualt settings, and the changes will be reflected on the frontpage of your website.
There are hundreds of other options you can explore in the gantry framework, I would suggest just clicking around on a test wordpress setup to see the extent of how you can customise your site using the framework.