please help blank pages coming up at checkout

i have just set zen cart and got everything how i want it up ive gone to test the payment procedure and when i go from my shopping cart to checkout it comes up with a blank white page and i cant do anything have i missed something i have only chenged the style sheet to fit the logo on and had someone help me that new what they are doing anything else i have not touched

i dont want to have to redo everything as this hapend before so i uninstalled it and re did it

thankyou

Try
http://www.zen-cart.com/forum/announcement.php?f=2&a=12

i put the debug pack in and this is what it came up with in the debug erroro log

PHP Warning: Cannot modify header information - headers already sent by (output started at /home/content/u/n/t/untamed69angel/html/store/includes/languages/english.php:1297) in /home/content/u/n/t/untamed69angel/html/store/includes/init_includes/init_templates.php on line 78

how do i fix it

thankyou