Q:

How to make method in data access class for parameterized query?

I have created one method in data Access class to select data from database with parameter. I just want to use parameterized query.
Method Is :
public DataTable executeSelectQuery(String _query, SqlParameter[] sqlParameter)
            {
                SqlCommand myCommand = new SqlCommand();
                DataTable dataTable = new DataTable();
                dataTable = null;
                DataSet ds = new DataSet();
                try
                {
                    myCommand.Connection = openConnection();
                    myCommand.CommandText = _query;
                    myCommand.Parameters.AddRange(sqlParameter);
                    myCommand.ExecuteNonQuery();
                    myAdapter.SelectCommand = myCommand;
                    myAdapter.Fill(ds);
                    dataTable = ds.Tables[0];
                }
                catch (SqlException e)
                {
                    return null;
                }
                finally
                {
                    myCommand.Connection = CloseConnection();
                }
                return dataTable;
            }

but I can't understand how to use this method to fetch data and how to pass parameter? 
My query may be "select password from tblUsers where email=@email" How to pass @email at business layer?
How to make method in data access class for getting Scalar value?
public string getpasswrd(string unm)
    {
        con.Open();
        string cpaswrd;

        cmd1 = new SqlCommand("select password from tbl_login where username='" + unm + "'", con);
        cpaswrd = (String)cmd1.ExecuteScalar();
        con.Close();
        return cpaswrd;

    }

A:

    SqlParameter param;

    cmd1 = new SqlCommand("select password from tbl_login where username=@username, con);

    param = new SqlParameter("@username", SqlDbType.VarChar);
    param.Direction = ParameterDirection.Input;
    param.Value = unm;
    cmd1.Parameters.Add(param);

    cpaswrd = cmd1.ExecuteScalar().ToString();

