You will need a DataGrid click event (DoubleClick or maybe CellClick) and show your popup as a dialog > ShowDialog()
private void DataGrid_DoubleClick(object sender, EventArgs e)
{
PopUpForm popupform = new PopUpForm(Pass Parameter);
popupform.ShowDialog();
}
Parameter you are going to pass should be defined on your PopUpForm like
public PopUpForm (type Parameter)
{
InitializeComponent();
parameter = Parameter;
}
Hope it helps