customise post order by custom field for certain categories

i have a certain number of my categories that have a custom field called “start date”. (Which is a date field). When i display one of these categories as per normal.

ie sitename.com/category/categoryname

i want these categories to display in “start date” order and not publish order? In date order ie two posts one has a start date of 09022013 and the other is 09052013. The 09022013 would display first.

I have done some research and I think I have to add some code to the category.php file specific to these categories and sort by custom field “start date” but im not so sure.

Any advice would be appreciated.