How to make a search form?

When you code get_search_form() WP search searchform.php in your theme root and print all inside that file.
And you can create searchform.php inside your theme root and insert this:

    <form role="search" method="get" action="<?php echo home_url("https://wordpress.stackexchange.com/"); ?>">
        <fieldset>
            <input type="text" name="s" value="<?php the_search_query(); ?>">
            <button></button>
        </fieldset>
    </form>

That basic search form.
Then code <?php get_search_form() ?> where you want this form in your theme. You can add your own class or ID inside that form and change what you want. Also I put link for couple notes about search_form