Hi,
There are so many ways to do this .some of developer follow one method and another follow another methods
one method is storing username in session variable then use that session variable in any page.
Here 'm providing link for session
http://msdn.microsoft.com/en-us/library/ms178581.aspx
After creating session variable we use that variable in login method after complete verification of user
Let me Tell you the process of your requirement
frst step is creation of session variable write the below code in session start event in global.asax
Session.Add("UserName", "");
After that write below code in login button
Session["UserName"]=TxtUname.Text;
After this you can use that session variable in any page by using following code
LblUserName.Text=Session["UserName"].Tostring();
I hope you understood What I said
All the Best