Add the handler to OnLoad function of the form

 protected override void  OnLoad(EventArgs e)
{
 	 base.OnLoad(e);

//Add Selection changed handler
    dbGridView.SelectionChanged += new EventHandler(dbGridView_SelectionChanged);

}


//Whenever the row selection is changed, the text box is filled with the Column 1 data

private void dbGridView_SelectionChanged (object sender, EventArgs e)
        {
        
                     DataGridView dgv = (DataGridView)sender;    //User selected WHOLE ROW (by clicking in the margin)   

            if (dgv.SelectedRows.Count > 0)
                textBox1.Text =  (dgv.SelectedRows[0].Cells[0].Value.ToString());

            //User selected a cell (show the first cell in the row) 
            if (dgv.SelectedCells.Count > 0)
               textBox1.Text = (dgv.Rows[dgv.SelectedCells[0].RowIndex].Cells[0].Value.ToString());

            //User selected a cell, show that cell 
            if (dgv.SelectedCells.Count > 0)
                textBox1.Text =(dgv.SelectedCells[0].Value.ToString());
        }