Subdomain: How to set up codeigniter??

[eluser]Buso[/eluser]
Hi, i have a site developed in plain php (no frameworks or mvc), and I would like to add a subdomain, of a blog made with codeigniter.
Is there something I should set up specifically for this case?? When I go to my subdomain, eg:
myblog.mydomain.com

I get blank pages with weird messages like "No input file specified." or "directory size: 1234" =/