The easiest way would be to put conditional tags around your sidebar saying that it should only show up if you are NOT viewing a page.
Here is the Codex’s list of conditional tags. In your sidebar.php template, you would want to add these tags somewhere near the top, probably right under where it says
<div id="sidebar">
<ul>
but before the actual *content* of the sidebar starts. Then just add the tags for all the pages you want the sidebar to show up on. It will look like this:
<div id="sidebar">
<ul>
<?php if (is_home() | is_single() | is_category()) : ?>
(or whatever “if” statements you choose)
…
sidebar content
…
then at the bottom of all the sidebar content you’ll add
<?php endif; ?>