Deploy a WCF-Service on SharePoint

Hi,

I want a WCF Service on a SharePoint, so everybody who uses the SharePOint can access this Service. Also a Silverlight-Application which is implemented in the SharePoint should use the WCF-Service.

How can I get this WCF Service on the SharePoint?

I read a lot  of tutorials in the internet. Everytime it says to create a SharePoint-Project in Visual Studio and so on, but I don't understand how that should help.

The SharePoint already exists and I just want to put somewhere there the WCF Service.
If its is neccessary for understanding: The SharePoint is located somewhere on a company server.

Any help is appreciated.

Thanks