Su error

[monteiro]

Whenever I try to do commands which require root previlage from thr KDE I receive the following error
"Su returned with an error".

A quick responce would truely be appreciated

[Harry Kuhman]

Set one, then use it. Read this.

[monteiro]

I have set a root password already.
When I do
su
kcontrol
Error about XLib refusing connection.
What next?