newbie to WP need help

Hi Im new to Word Press. I have used php and mysql for small websites which i have created myself, but have never looked into using this to help build sites. Im helping a friend with her website which uses WordPress for the shopping cart section of her site.

I was mainly looking at the front end and leaving the rest as is, but then i tried to validate the site and it came back with a whole load of errors. Im used to editing the php code within the page to eradicate the errors, but i cant seem to find the pages to edit. Am i being silly or looking in the wrong places? I have also read in some of the forums that the plugins can cause errors so how do i look at the code for the plugin and edit it?

The website that is not validating is http://bazaar.lalesso.com/

I would be very grateful for any help

Thank you