Here's a function that will give you the row number, I have also included a proc that shows how to call the function.
VB Code:
Function FirstEmptyRow(ByRef SearchSheet As Worksheet, ByRef SearchCol As Long) As Long
Dim rngCell As Range
Set rngCell = SearchSheet.Cells(1, SearchCol)
Do While rngCell.Value <> ""
Set rngCell = rngCell.Offset(1, 0)
Loop
FirstEmptyRow = rngCell.Row
End Function
'Here's a sample showing the function in action
Sub testempty()
Dim x As Long
For x = 1 To 4
Debug.Print FirstEmptyRow(ThisWorkbook.Worksheets(1), x)
Next x
End Sub