absolute path

[eluser]Unknown[/eluser]
Hi there,
I have an application that works perfectly in my localhost, but since I uloaded it in my domain it gives me "404 page not found" and "disallowed key characters" errors. I think it's the configuration. What should I put in "base_url"? I put http://www.mydomainname.com, but where may I define the absolute path?
Thanks in advance.
Bebo