categories and posts

Hi

I am trying to get my sidebar to list my posts under a specific category. When I use the widget Categories it is only listing the categories.

How do I get my sidebar to list the category and then under that category list the posts associated with that category. So if I have the category Beginning and a post title start here how can I get my sidebar to reflect this.

Then another category titled advanced and under that a posts more info

All I want is for the sidebar to name the category and then under the category lists the posts that are associated with that category.

Is there a plug in that will do this?