How to pass a variable value to the sql services reports

I am calling the report via asp.net page: just by calling the following url:

How can i pass a variable value which has username information in it:

Please help me, i need to pass a username, which is in session("Username"),

On the report rptStatSU, i am calling a stored proc. so would like to pass the username value, from .net [roject directly to sql server reporting services:

Dim Url As String = "http://localhost/Reports/Pages/Report.aspx?ItemPath=%2fCCSnetReports%2frptStatSU"

Please i am new to reporting services and struggling to learn.

Thank you very much for the information.