How do I use wc() in woocommerce?

as what the doc in your link says. 'prevent the need to use globals'. An example would be like this...

code using global.

global $woocommerce;
$customer_country = $woocommerce->customer->get_country();

code not using global

$customer_country = WC()->customer->get_country(); 
// some servers may not like like this... best is to use variables like $foo = WC(); then use $foo->customer->get_country()...

How to use WC() ? start here...

why must I avoid global?