Column Insert Help

Hi,
I have some values I want put into a table, but the values are from other sources and I dont know how to retrieve them..
I'll
show my code, and the bold is explaining what I want inserted and where
from. I'd apprechiate if someone could help me with syntax etc. There
are 2 about getting value from another table and one about just putting
in straight forward text..:
 
        command.CommandText =
"INSERT INTO Messages (sendername,recievername,message,Date,subject)
VALUES (@sendername,@recievername,@message,@date,@subject)";

command.Parameters.Add("@sendername", System.Web.HttpContext.Current.User.Identity.Name)
command.Parameters.Add("@recievername", every value of column named Usersname of the Transactions table, WHERE Itemid=Itemid in the gridview on this page );

        command.Parameters.Add("@message", the value of items table - column 'paymentinstructions' WHERE Username=System.Web.HttpContext.Current.User.Identity.Name);

        command.Parameters.Add("@subject", some text: IMPORTANT - Payment Required);

        command.Parameters.Add("@date", DateTime.Now.ToString());

        command.ExecuteNonQuery();
 
Thanks alot if anyone can help me with those three things..
 Jon