From your administration page, go to "Design"->"Theme"-"Layout". From there, you can choose the page you want to edit. After that, it's as simple as clicking and dragging the features you want displayed in your Sidebar or Body! Once an item is in your sidebar or body box, you can customize it by clicking the little down arrow.
You can also add your own, fully customized sidebars. Drag "My Text Blocks"to your sidebar or body just as you would with the preset items. When you click the little down arrow you will see a text box in which you can type whatever you want, including HTML code. These boxes are great for announcements, embeddable audio players, Facebook and Twitter buttons, and so on.