Loops that pull from specific categories

What code do I add/change in the following basic loop so that it will only look for posts in a specific category?

<?php
get_header();
if (have_posts()) :
while (have_posts()) :
the_post();
the_content();
endwhile;
endif;
get_sidebar();
get_footer();
?>

Also, what code do I need to add (if any) so that I can follow this loop with another loop, looking for posts in yet another category?

Thanks