Call to undefined function get_currentuserinfo()

Hi everyone, I am a newbie programmer and I really need help.

I have a wordpress site, and I want in a .php page of my own to display the info of the logged in user.
I tried this:

<?php
 global $current_user;
 get_currentuserinfo();
 echo $current_user->user_login;
 ?>

but I get a constant error “Call to undefined function get_currentuserinfo()”. I included the “pluggable.php” file which contains the function but it didn’t work.
Somewhere I came across this which might be the solution:
`Apparently this is happening because the file /wp-includes/pluggable which contains the function doesn’t get loaded until after the plugins are loaded. Indeed it is. So wrap whichever thing you’re doing in a function, and hook it onto the plugins_loaded or init hook. (see the wp-settings.php file)

What exactly do I have to write and in which file?