how to get the row index value in datagridview

[C# Programming]

hi i have a form name frmDataEntry with textboxCustID and textboxCustName and lookup form name frmCustAssist with texboxFilter, button Filter and datagridview then when i double click textboxCustID then frmCustAssist will show i have to look if customer already exist or not then if already exist i hav to get the value of row index ..then the custID and custName of that value selected will be fill in the formDataEntry in textboxCustID and textboxCustName how do code that that plz help

my code is here..

in a frmCustAssist

private void button1_Click(object sender, EventArgs e)
{
bindingSource1.Filter = "LASTNAME like '%" + textBox1.Text + "%'";
bindingSource1.Sort = "LASTNAME";
selected row value .. somwthing like that

}
in a frmDataEntry

selected value custID =textboxCustID
selected value custname = textboxCustName