[dba-SQLServer]Update Trigger

I need to create an update trigger on a table to update a field in another
table.  This is the scope:

Originating table: 	System  /**I did not name it, proprietary package**/
Field:			CS_No 
Key Field:		Site_no

Destination Table:	Recur_Line
Field:			UDF1CS_no
Key Field:		Service_Site_No

Scenario:  When the user is in the system, he enters in a CS_No in the
table(System) field(CS_No).
I need this to populate the UDF1CS_no field in the Recur_Line table

when the System.Site_no=Recur_Line.SiteNo

This is my first "cross pollinating" trigger as I refer to it.  Does anyone
have an example?  Thanks, gurus.		
		
Karen Nicholson
SQL Database Administrator
Guardian Protection Services