How do I show username & email address of currently logged in user on a page he is viewing?
You can create a block with the PHP code format (don't forget to activate the PHP filter module) and put inside: 

&lt;?php
  //Retrieve the global variable for the connected user
  global $user; 
  echo 'Hello ' . $user-&gt;name;
  echo 'Your email is :' . $user-&gt;mail;
?&gt;


Then you set up this block to show only for authenticated users and you place it in a region.

See Drupal global variables documentation.