Can anyone help me out how to use session concept..

My scenario is like this..There is username and password page(Login.aspx).. When user types the username and password( C# should check whether the username and password is there in SQL Server table), it gets logged into the website with current date and time being displayed at right top side of the page..Username should be there till the user sign out's from the page..I would be glad if anybody helps me out this session concept according to my scenario..