Hi,
You will need to include an event macro to accomplish that.
Try right-clicking on the tab of your sheet and select "View Code"
Paste this code in the editor.
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Application.Intersect(Target, Range("A1")) Is Nothing Then
If Range("A1").Value = "No" Then
Range("A2").Value = 0
Else
Range("A2").Value = ""
End If
End If
End Sub
Now go back to the sheet and test.