Increment Cell Range

[mef1sto]

Maybe someone could give me a helping hand with this code right here:

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
  If Not Intersect(Target, Range("H13")) Is Nothing Then
      Run "testtwo"
  End If
end sub
'In a module
Sub testtwo()
Application.EnableEvents = False
Dim lastc As String, j, firstc As String, sstep
Dim msgTrap As VbMsgBoxResult
Dim newTP As Variant
   ':warning: SO I WANT TO INCREMENT THE DEFAULT NUMBER FROM RANGE B SO THAT THE CELL NUMBER INCREMENTS BY 1 IF THERE IS DATA IN THE LAST UNEMPTY ROW OF COLUMN B
'I FORGOT TO MENTION THAT THE FIRST CELL TO ENTER THE DATA IS RANGE("B16") AND THE END CELL IS RELATIVE
' SOMETHING LIKE IF IN CELL B18 I HAVE DATA THE NEXT TIME I DOUBLE CLICK RANGE h13 MY NEXT DEFAULT RANGE WOULD BE B19 AND NOT STILL b16 AS IN MY CODE RIGHT NOW....HOW DO I DO THAT?!
 msgTrap = MsgBox("Insert new data?", vbYesNo + vbQuestion, "?")
    Select Case msgTrap
    Case vbYes
    firstc = InputBox("Please insert the cell to store the data eg. B16", "!!!", Default:=Range("B16").Address(0, 0))
        If Len(firstc) = 0 Then Exit Sub
        newTP = InputBox("Insert the new data!", "Update!")
        Range(firstc) = newTP
        End If
        If newTP = 0 Then
            newTP = ""
            Range(firstc) = newTP
        End If
            Range("H13") = newTP 
    Case vbNo
        Exit Sub
    End Select
End Sub