How to get UserId of a logined user from ASPNETDB.MDF?

Question

How to get UserId of a logined user from ASPNETDB.MDF? I have create some users through Web Site Administration Tool in ASP.NET 2.0. After a user logined, I hope to add some records to a table which include UserID field for the logined user, but I don't know how to get UserId of logined user, and how to insert the uniqueidentifier field UserID to a table?