How to show the posts of some category first, and then the rest? Cant figure out the coding

You could use 2 different loops: 1 for some of your categories and another for the rest.

//Here 2,6 are the category ids
$query1 = new WP_Query( array( 'category__in' => array( 2, 6 ) ) );
// The Loop
while ( $query1->have_posts() ) {
    $query1->the_post();
}
// Restore original Post Data
wp_reset_postdata();

$query2 = new WP_Query( array( 'category__not_in' => array( 2, 6 ) ) );
// The Loop
while ( $query2->have_posts() ) {
    $query2->the_post();
}
// Restore original Post Data
wp_reset_postdata();

Use this code on the page where you would like to show your category posts. If your’re new to this,maybe you should first know about template hierarchy to figure out which page is being loaded and in which file you should place your code.