Both of these features can be change via the presentation submenu.
To change your theme, go to your dashboard, select presentation and try-out any new theme available. For more help go to your WordPress Tutorial.
WordPress Widgets is like a plugin, but designed to provide a simple way to arrange the various elements of your sidebar content (known as “widgets”) without having to change any code.
The Widgets SubPanel explains how to use the various Widgets that come delivered with WordPress,To add widget, scroll down and make selection from what’s available and drag it to where your heart desires.
As usual, for more updates and details go to the WordPress tutorial, the FAQ or the WordPress manual all made available via my blog at the right hand side panel.