Please be more specific while asking a question.
What you have tried ? Provide at-least some code snippet so that we can figure out where you are making a mistake.
Assuming you are writing your own code to validate the user credentials,
after checking the credentials if they are valid please place the corresponding username in session by
session["name"]=username(may be username textbox.text)
wherever you wanna access it
lblUsername(your label name).text=session["name"].toString();
Hope this helps