WordPress is making great strides as a CMS, but it does lack one feature found in most CMSs: the ability to control over which posts and pages your widgets appear on.
The Widget Context plugin is the best and most usable solution. It consists of a not much more than some radio buttons, some checkboxes and a box for URLs.
Step 1. Find and install the plugin
Step 2. Go to your widgets page
Step 3. Select the widget to work with
Step 4. Configure your choices
Option: Type in the target URL. Put only one URL per line. You can use an asterisk as a wild card to apply the rule to an entire category, a year, a month or just a single page. The note under the box gives you examples:
Click Save. That's it. You're done. Just experiment to get it right.