You can take the user ID/password and compare them to what you have in the database. If an item is found that is matching, grab the user name (ie, David) from the database, and display the name in a label. If the name is something that you want to persist across pages, then you could store the name in a session variable and then display it on several pages across the app.

int iUser = 0;
SqlDataReader reader = myCommand.ExecuteReader();

while(reader.Read())
{
myLabel.Text = reader["FirstName"].ToString();
// Session["UserFirstName"] = reader["FirstName"].ToString();
}

if (iUser == 0)
// in valid user id/password
else