how to make "wp_get_current_user();" work?

I download a plugin (Login to view all) ,and want to modify it furthermore.
What I want to do is find out who is on the site and he is Not a Customer.
( I also download an e-commercial plugin,which would make a new roll called “customer” )

So I edit the plugin(Login to view all) .
The plugin only have one php file.
So I add the code

wp_get_current_user();
$iden = $current_user->ID;
global $wpdb;
$str = $wpdb->get_var($wpdb->prepare("SELECT meta_value FROM $wpdb->wp_usermeta WHERE meta_key = 'wp_capabilities' and user_id = $iden ;"));
$rollplayer = unserialize($str); //To see what the roll is.

and in the if statement , I add $rollplayer!==customer to ask a more specific situation.
But some problem happened.
I got
Fatal error: Call to undefined function wp_get_current_user() in C:\xampp\htdocs\wordpress\wp-content\plugins\login-to-view-all\login-to-view-all.php on line 14
So, I comment out the wp_get_current_user();
And there is no error .
But it seems I can’t get the $iden .
Of course ,I can’t get the $rollplayer.

How to make the code work?
Should I add something more?

Thanks in advance.