Execute code and get user name from server side

Hello to everybody

I started programming in silverloght a few weeks ago.

I created my project starting with the standard "Silverlight Business Application", already present in VisualStudio 2010.

There is some code(i created a C#class) on server-side that i want to be executed when the user logon, and in that class i need to have the "UserName" of the who is logged in(remember that i am on server-side).

how can i do this 2 things??

thanks