how to show only specific category for a template

You can do that by using category parameter in the wp_query

change the line which looks like this

$blog_query = new WP_Query( array( 'post_type' => 'post', 'paged' => $paged ) );

to

$blog_query = new WP_Query( array( 'post_type' => 'post', 'paged' => $paged, 'cat' => 4 ) );

// Here ‘4’ is the id of category you want to show, whereas you can also use the category name

For details see this link: http://codex.wordpress.org/Class_Reference/WP_Query#Category_Parameters