enable a groupbox when we select value from combobox

Whenever a value is selected from a combo box the following `SelectedIndexChanged` event triggers. So, you should try something like this:

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
      ' TODO : Here you can also set index based criteria
      ' If (comboBox1.SelectedIndex = 2) Then
      ' groupBox1.Enabled = True
      ' End If

      groupBox1.Enabled = True
End Sub

Visit MSDN documentation for `ComboBox` control as well.

Hope it helps!