You’ll need to edit some code in order to get rid of the sidebar. Depending on what pages you want to remove the sidebar from, you might have to edit multiple files.
The code you are looking for is:
<?php get_sidebar(); ?>
This is the code that calls the sidebar file. This will be found close to the bottom of page.php, index.php, single.php, archive.php, 404.php and search.php. Again, depending on how your site is set up, you might only need to delete the code from a few of the files or you might need to delete it from all.