You can't just go:
SqlDataReader reader = command.ExecuteReader();
string selectedStudentName = Convert.ToString(reader);
You need to actually read the values from the db, and specify which column you want as weel as (as Mike Meinz says) using a parametrized query.
Try this:
string str = "select * from tbluser where flduser=@USER";
SqlCommand command = new SqlCommand(str, con);
con.Open();
command.Parameters.AddWithValue("@USER", TextBox1.Text.Trim());
SqlDataReader reader = command.ExecuteReader();
string selectedStudentName = "";
if (reader.Read())
{
selectedStudentName = (string) reader["NameOoColumnWithUserNameInIt"];
}