Find Next Empty Cell

Hi,

I would like some help please to find next empty cell. My cells are C18, H18, C33,H33, C48 and H48. I would like a code that first looks at C18 and if its empty insert here if not then H18 if thats empty insert here if not then C33 ...and so on to H48.

I started to create the following code (not complete) but I'm sure there must be better way

Private Sub CommandButton1_Click()

If Range("C18") = "" Then
Range("C18").Value = TextBox1.Value
Else
If Range("H18") = "" Then
Range("H18").Value = TextBox1.Value

End If
End If
Unload Me
End Sub