How to remove Powered by Drupal?
Drupal constructs the pages you see using blocks and nodes through a template file. To find that text, look in menu option "website information", next if not found look in the block list for a block placed in copyright or footer area. 

Drupal will depend on block/regions; learn that too. Finally, when that doesn't work, in /sites/all/themes/ you'll find the PHP and HTML files delivering the themed page.