Main page displaying post from certain categories?

You’d need to use a custom query such as:

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$args= array(
	'cat' => -4,
	'paged' => $paged
);
query_posts($args);
?>

where 4 was the id of the catgeory you wanted to exclude.

https://codex.www.ads-software.com/Template_Tags/query_posts#Category_Parameters

Thank you,

Where would I put that code?

In your theme’s index.php template file.