How can I get the first post THIS SAME CATEGORY?

You can use additional WP_Query parameters like post_per_page

For example,

$my_custom_query_args =    

    array( 'posts_per_page' =>1,

       'orderby'=>'DESC'); 

$my_custom_query = new WP_Query( $my_custom_query_args ); 

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)