Tælle i taller klik på label

Dim a As String
Private Sub Form_Load()
a = 1
End Sub

Private Sub Label1_MouseDown(Index As Integer, Button As Integer, Shift As Integer, X As Single, Y As Single)
If Button = 1 Then
Label1(Index).BackColor = Black
taller.Text = a
End If
End Sub

Hvad skal jeg gører for at den skal tælle i taller