Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search gave over a million hits, just for C#!
Google[
^]
Following a random one of them gives you at least one solution...
http://www.dotnetfunda.com/forums/thread4397-how-to-display-username-in-a-lable-after-he-logins.aspx[
^]
In future, please try to do at least basic research yourself, and not waste your time or ours.