Create Relationship with cardinality between two table in .MDF

Dear all, i'm using Visual Studio Express 2010 and i've created a DB in Visual Studio with Sql Server. I've had a simple problem, i don't know how to set the cardinality to relationship. For example, i've 2 Table User and Site in my Database diagram. Clicking with right button, i go in relationship. Here i add a relationship between User and Site, without i can to set the cardinality, so Visual Studio creates a one-to-one relationship. Can i resolve this problem? Thanks a lot! ...