How to make default value for combobox with selectedvalue=0

Question

Hi i work in visual studio 2015 with c# windows form

i need to assign default value with value 0 and text select to combox

but How can i do that ?

In windows form load i make 

 var fileName = string.Format("{0}\\Book3105", Directory.GetCurrentDirectory());
            var connection = string.Format("Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source={0};Mode=ReadWrite;Extended Properties=Excel 12.0 Xml;", fileName);
OleDbConnection con = new OleDbConnection(connection);

            try
            {
                con.Open();
                str = "select * from [sheet2$]";
                com = new OleDbCommand(str, con);
                oledbda = new OleDbDataAdapter(com);
                ds = new DataSet();
                oledbda.Fill(ds, "[sheet2$]");
                con.Close();

                dt = ds.Tables["[sheet2$]"];
                int i = 0;


                for (i = 0; i <= dt.Rows.Count - 1; i++)
                {

                    comboBox4.Items.Add(dt.Rows[i].ItemArray[1]);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

I can get data from Excel 2007 xlsx file but cannot assign default value --select-- with selected value 0

How to do that if possible