You will need to modify the template-blog.php file located in the theme root folder. Here is the code change –
$query = array('cat' => '-3,-6', 'posts_per_page' => intval(get_option('posts_per_page')), 'ignore_sticky_posts' => 0, 'paged' => $paged);
located at line 23 of the above mentioned file. You will need to specify the cateory IDs for the ‘cat’ argument. Thanks