This would be easier if you use master page in your project , and a dedicated label to display the username in the same master page.
After successful login you can pass the username to a session and pass this session value to the label text.Refer the below code;
if login is successful add the following codein your logic
Session["username"]= txtusername.Text;
And in the master page page load event.
lbl_user_name.Text = Session["username"].ToString();
In this way you can see the username in all the pages in which you are applying the above used masterpage.